StealthSonics C9 Pro – Silky dream!

If you're looking for an IEM that is highly engaging and smooth with a strong bassline, full mids with superb vocals and very refined and elegant treble then the C9 Pro is definitely among the top options especially at its price point! If you like your organic signatures with a bit snappier and punchier bass presentation, clearer mids with more bite and energetic treble then there are other alternatives as we'll see in the comparison section.


  • Full bodied, richly textured and silky smooth midrange
  • Exquisitely refined treble presentation
  • Beautiful vocals with excellent presence (powerful delivery for male and sweet delivery for female)
  • Powerful bass line with great slam factor
  • Very coherent and open soundstage with precise imaging
  • Excellent resolution
  • Superb build quality and perfect fit
  • Competitive price


  • Might be too warm and full bodied for some

Manufacturer’s website :
Price : 1499$

Fit, Build & Isolation

StealthSonics is no newcomer to building IEMs and the C9 Pro build is simply superb, the shell is flawless and the smoke transparent show great craftsmanship on the internals. I picked simple colored transparent faceplates but the result is really nice and I love looking at them as much as I love to listen to them. The 2 pin socket is recessed, which has my preference for a more secure fit.

As you can see there are 3 sound tubes and bores are flush, which means be careful with was obstructing bores. I admit I prefer horn bore designs (like the EM10 or VE8) for that reason but it’s not widely spread yet.

StealthSonics C9 Pro build is flawless and shows perfect craftsmanship, fit is superb as well

For those of you that are used to customs, note that the fit is on the relaxed side (special thanks to Daniel from THL for his input on this) so if like me you like your fit a bit tighter with lower tolerances then don’t hesitate to ask.

StealthSonics took my now usual digital prints and the build time was pretty fast around 2 weeks, the result was perfect from the get go the seal is great as well as comfort. This is top notch custom experience.


The folks at StealhSonics are a « group of audiologist, engineers and musicians that have been serving the audiology and audio needs of musicians, audio professionals, audiophiles and patients in SouthEast-Asia for almost 10 years ». I confess I hadn’t heard about them before I took part to the U2, U4, U9 review tour but I am glad I had the opportunity to discover the brand.

The C9 Pro is the upcoming flagship (it’s actually not released yet) from StealthSonics along with the C9 with both a different tuning and technologies. The C9 was a hybrid with a dynamic and 8 BA while the C9 Pro is a 9 BA setup. StealthSonics does quite a bit of R&D and has its own techs implemented in the C9 Pro Klarity valve to reduce pressure build up and fatigue in the ear canal from long usage; Stealth damping for more controlled bass response ; and SonicFlo Acoustics for resonance and cancellation reduction.”

In the lineup I really liked the U9 which is ruler flat reference and loved the more organic U2 as it fits my personal preferences in terms of signature. Basically I liked the U9 technical chops but the signature of the U2 stuck with me. When I heard StealthSonics was coming up with a flagship with a very organic tuning my interest was piqued to say the least and I jumped into the opportunity of getting it early!


From the very first minute listening to the C9 Pro it was apparent that it was everything I hoped for and then some… the C9 Pro is the epitome of an organic signature to me, which is a full bodied, richly textured, full of nuances and smoothly delivered sound. But it doesn’t stop there, as we’ll see and that’s what sets it apart.

The bass line is powerful with a clear mid bass focus, and the C9 Pro has a lot of slam factor. Mids are full bodied with beautiful textures and a smooth delivery yet very open and articulate. Treble is exquisitely tuned combining excellent lower treble sparkle and energy along with excellent upper treble extension providing air and resolution as well as balancing the overall signature. The C9 Pro is organic, but it’s neither dark nor congested quite the contrary. Soundstage is excellent and very coherent making for a very precise 3 dimensional image. The overall coherence is flawless, and reflects a masterful tuning.


The C9 Pro bass is well extended and controlled but with a clear mid bass emphasis – the delivery is on the smoother side – with a soft attack and a tad longer decay. Bass notes linger a bit and decay beautifully.

It’s not a snappy, punchy bass but rather a more romantic delivery with beautiful textures and a high level of detail and very good control. Don’t mistake this for a weak or slow bass, the C9 Pro has a powerful bass line and it can keep up with faster paced music it only does so with a smooth delivery at all times. This is a fairly singular presentation among all the IEMs I have listened to and I must say I like it especially on percussion where the C9 Pro conveys nuances of interpretation in a way that not many IEMs can.

On José James “Better Off Dead”, Nate Smith drums are exquisitely rendered and and Ben Williams bass line is powerful with a lot of weight to the notes. In Lucy Dixon “Stormy Weather”, David Aknin drums also shine with the C9 Pro lots of nuances and interpretation cues. I also loved the way the C9 Pro presents the double bass on Jazz tracks, rich, soulful but some might find they prefer a snappier presentation. I like that the C9 Pro offers something different from most IEMs there and Caro Emerald “Back it up” sounded just as good as I have heard it.

Out of more powerful sources, like iFi Audio ICAN Pro or iBasso DX220 Max, it’s very apparent that the C9 Pro will scale yet a bit further with a more powerful source providing even better control and more slam factor.


The mids are the heart and soul of the C9 Pro, it’s a full bodied, richly textured and smooth mids with beautiful vocals.

The lower mids are probably among the very best I have heard elevated enough to provide satisfying body but not so much so that the mids would become congested. It’s also one very distinctive item of the C9 Pro’s tuning in a day and age where a common way to gain clarity is dipping lower mids. As we’ll see in the treble section, the C9 Pro relies on other qualities to bring clarity. This lower mids tuning highlights fundamentals on the rythm section of tracks and the C9 Pro has a strong rhythmic emphasis which makes for great toe tapping factor along with its mid bass emphasis. It also makes some older albums (80s come to mind) more enjoyable. Coherence between the bass and mids is also a remarkable quality of the C9 Pro’s tuning, while there is no bass bleed into the mids the transition is seamless.

The upper mids on the other hand are on the smoother side, there is good percussive attack and electric guitars have great bite but above around 5kHz there is a dip and some might find hi hats to lack crispness, kick drums and toms to lack a bit of snap and brass instruments are on the safer side. On the flipside this makes the C9 Pro non fatiguing and tracks like Hank Levy “Whiplash” or Miles Davis “So What” are absolutely devoid of any agressive hints that have me reach the limit with some IEMs.

Last but not least vocals on the C9 Pro are special, thanks to the fullness of the lower mids male vocals have power (Leonard Cohen “Nevermind” never sounded so good) and female vocals are on the sweeter side but it’s not overdone. This makes for a romantic presentation so to speak. The presence range is elevated and there is vocal emphasis, and the upper mids being on the safer side there is no hint of sibilance whatsoever. As we’ll see to bonify things the upper treble brings welcome air to an otherwise sweet and romantic presentation, making the C9 Pro a superb vocal performer in my opinion as it also emphasize higher notes and James Blake “Vincent”, Etta James “At last”, Ruth B. “Lost Boy” are some of my vocal test tracks I enjoyed a lot with the C9 Pro.

Superb transparent smoke grey shells on the C9 Pro


The C9 Pro treble, as I hinted earlier are key to its signature. The power of its mid bass and its full bodied mids is balanced by the energy of its lower treble and its superb extension and presence. Consistently with the rest of the signature, the C9 Pro delivers its treble section with a smoothness that is so very elegant to my ears.

The lower treble features excellent sparkle and piano notes have great energy and weight, the Hot Sardines “Comes love (l’amour s’en fout)” intro is simply superb. In a very different genre Infected Mushroom “Jeenge”, Erez Aizen acoustic guitar is superb as well as the airy synth sounds. Also worth mentioning, Stan Getz “Maracatu-Too” where Jose Paulo and Luiz Parga percussions are simply beautiful.

Uncharacteristic for a very organic IEM, the upper treble has excellent extension and presence, which brings a lot of air and refinement to the C9 Pro signature but also grants the C9 Pro impressive resolution and detail retrieval as well as excellent separation. This is even more apparent out of DX220 Max versus the smoother LPGT where the C9 Pro reaches impressive resolution. The upper treble presentation reminds me a lot of Vision Ears VE8 and that’s not a small compliment. It’s a very effortless and consistent across the range – again – smooth presentation making for a very natural presentation, refraining from any artificial accentuation which is the sign of a masterful tuning.


The C9 Pro is without a doubt my favorite IEM from StealthSonics and has quickly earned a place as one of the top favorites in my collection along with Custom Art FIBAE7 and Earsonics EM10. It’s a daring choice in this day and age for a flagship to go for a more organic signature but StealthSonics has delivered with the C9 Pro a flagship that is both technically impressive and a masterful organic tuning with its own unique twist.

If you’re looking for an IEM that is highly engaging and smooth with a strong bassline, full mids with superb vocals and very refined and elegant treble then the C9 Pro is definitely among the top options especially at its price point! If you like your organic signatures with a bit snappier and punchier bass presentation, clearer mids with more bite and energetic treble then there are other alternatives as we’ll see in the comparison section.

Note that for those of you who prefer universals the U9 Pro will also be available.

Listening notes
I spent approximately 300 hours with the C9 Pro , listening on Lotoo PAW Gold Touch, DX220 Max and PAW S1 using PW Audio n°10, DITA Oslo and Dunu Blanche balanced cables. 

Special Thanks
Thanks to Raj at StealthSonics for providing an early preview to the C9 Pro at a discounted priceAs usual, this review is my honest opinion. No incentive was given for a favorable review. 


  • Leather carrying case
  • Stock copper cable with 3.5 termination
  • Cleaning tool


  • 9 balanced armature drivers
  • Impedance : TBC
  • Sensitivity : TBC

Select comparisons

StealhSonics U9
I don’t own the C9 but the U9 should be close enough especially with custom tips. This is an interesting comparison as it’s a coflagship in the StealthSonics lineup. Note that the U9 (and C9) are hybrids with a dynamic driver and 8 balanced armature drivers contrary to the all 9 BA setup for the C9 Pro.

Starting with the bass, U9 is significantly leaner in terms of bass presence compared to the C9 Pro but the two have a very different tuning. The U9 is a reference tuning with reference bass, the mid bass is tight and controlled and very detailed and quite fast for a dynamic. Worth noting, there is more power and rumble in the sub than the C9 Pro, the dynamic driver is definitely helping. The C9 Pro has more slam with a much more prominent bass line and overall much more bass presence.

The U9 mids clearly follow a very different tuning goal than the C9 Pro, with a clear focus on transparency and clarity and the U9 mids are very accurate with spot on timbre. The U9 lower mids are much leaner than the C9 Pro but there is good body nonetheless and similarly to the C9 Pro the upper mids region is safe. There is a bit more bite in the upper mids though and I love the U9 on Jazz. Compared to it’s older brother the C9 Pro is warmer with a pleasing tone that is north of accurate, and features a much fuller bodied and organic presentation that is not at the expanse of articulation which is impressive.

Treble might be where the U9 and C9 Pro are the most similar, with good lower treble sparkle and superb upper treble extension. The difference lies in tone, the U9 again is spot on reference with great accuracy while the C9 Pro has a warm hue that makes its treble sound natural and smooth. Another difference is the faster decay of the U9, with a longer note decay on the C9 Pro. Both the U9 and C9 Pro feature great air and resolution, with the U9 having a slight edge on imaging precision and soundstage by way of its more reference tuning.

Custom Art FIBAE7
The FIBAE7 has long been my favorite IEM, featuring another masterful organic tuning. It is, to me, the benchmark for organic tunings. How does it compare to the C9 Pro? Let’s check out.

The FIBAE7 and C9 Pro have a similar soundstage although the C9 Pro is a bit taller and deeper. The FIBAE7 does feature a snappier presentation with more bite on the attack and a bit quicker note decay. The C9 Pro on the other hand is smoother with a smoother attack making for a more romantic presentation.

The FIBAE7 has significantly more sub bass presence than the C9 Pro. This is very apparent both in Sohn’s “Falling” and Aphex Twin “Ageispolis” where the FIBAE7 provide a more visceral presentation that few balanced armature IEMs are able to convey. The FIBAE7 also feature a snappier attack in the mid bass section, making for more slam factor. On the flipside the C9 Pro has more mid bass emphasis making for a stronger bass line presence. The bass line has quite a bit more power on the C9 Pro with a more textured delivery where the FIBAE7 is tighter and snappier.

The FIBAE7 mids are a bit different from the C9 Pro : while both have an organic tuning the FIBAE7 has more bite in the upper mids there is more crispness in hi hats, toms have much more snap and brass instruments have significantly more bite making the FIBAE7 clearer and snappier than the C9 Pro. On the flipside the C9 Pro features more lower mids presence with a fuller bodied presentation and where the FIBAE7 is overall more neutral natural the C9 Pro is warmer.

Last but not least, the FIBAE7 treble lower treble is a bit more energetic than the C9 Pro with more sparkle, not that the C9 Pro lacks sparkle but it’s a bit less energetic. Both feature nice weight in the lower treble with good note impact, but the C9 Pro delivery is a bit smoother with longer decay. Now upper treble, I hear the C9 Pro having more upper treble presence and while it’s less energetic than the FIBAE7 it has a bit more air and shimmer.

Overall if I had to sump it up, the FIBAE7 is more physical, with overall more energy thanks to the punchier bass, upper mids with more bite and a tad more sparkle in the lower treble. The C9 Pro on the other hand has a stronger rhythmic bass line with richer textures, fuller mids with an overall warmer tone and more delicate lower treble with more air and refinement up top.

Earsonics EM10
The EM10 is the long standing Earsonics 10 BA flagship and for good reason. How does it compare to the C9 Pro?

The EM10 is another organic IEM in my collection and therefore a very interesting comparison to the C9 Pro.

The C9 Pro and EM10 share similarities in terms of bass. Like the C9 Pro, the EM10 feature a clear mid bass emphasis with a prominent bass line and a very satisfying slam. The EM10 is to this day my benchmark on mid bass, as I consider it the best mid bass a BA IEM can provide. Both the EM10 and C9 pro feature a longer than average decay in the mid bass with superb texture but I find the EM10 to be snappier than the C9 Pro with a snappier attack and better control. The EM10 also has significantly more sub bass presence with much better rumble providing additional physicality.

Like the C9 Pro, the EM10 lower mids have significant presence making for a fuller bodied presentation and both have exquisite seamless transition from the bass to the lower mids. I’d say the C9 Pro has a touch more body and is overall warmer than the EM10. Both feature slightly forward vocal placement but the C9 Pro is again sweeter. The upper mids are the clear differentiator : the EM10 has significantly more bite than the C9 Pro there, with more crispness and snap also making for a clearer and more energetic mid presentation where the C9 Pro is ever the refined romantic.

Now on to treble, the EM10 and the C9 Pro both offer a very natural and refined presentation, both offer good treble note weight with the C9 Pro having a bit more there. But again, the EM10 is more energetic with a tad more sparkle as well as a more accurate tone where the C9 Pro has a similarly engaging but more nuanced presentation with a warmer hue. Extension is superb on both and the upper treble is more similar than not, both provide excellent resolution and air.

Overall the EM10 and C9 Pro are two different takes on organic signature : the EM10 is the more energetic with snappier bass with more rumble, full but clearer mids with more bite and a tad more sparkle in the lower treble. The C9 Pro has a more nuanced and smoother delivery with an overall warmer tone and even fuller bodied presentation.

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