Stealth Sonics U2, U4 and U9

U9 – The clear one

  • Drivers: 1 x DD (Low), 8 BA (2 x Mid, 2 x High, 4 x Super-High)
  • Crossover: 4-way
  • Frequency response: 18Hz – 40kHz
  • Sensitivity: 108dB at 1mW
  • Impedance: 16 Ohms at 1kHz
  • THD: <=1% at 1kHz
  • Price: US$1099



Contrary to the U4, I do feel the U9 are closer to a flat response and I would call it close to neutral, but without becoming dry and loosing their musicality. The U9 are clear, clean and musical with a surprisingly natural tonality to instruments. In that sense there is something of the Rhapsodio Eden to them. The overall tone feels quite bright (in the good sense) and clear, yet instruments still maintain a good body and accurate tonality. Perhaps not quite at the level of the Eden, but those are nearly twice as expensive.

The U9 have a very nice spacious stage that feels immersive and I find works well for classical music. The background is not as black as I would like, but perhaps that has something to do with the Klarity Valve, as the contrast of the notes relative to the background still feels very natural. IEMs with a very black background can have a strong contrast that although it emphasizes details and textures, is not actually natural as such. Thanks to the spacious stage and the Klarity Valve, the U9 feel more like listening to headphones even though the stage is not as big. At times the low weight even made me forget I was wearing IEMs. It is a very natural way to experience classical music. The stage does seem to have a mind of its own and I find the imaging of one of my favorite albums, Caro Emerald’s Acoustic Sessions, to be off somehow and I have not been able to put my finger on it. Other jazz, like my latest favorite track, Tuba Skinny’s Jazz Battle, does feel better, so it could be down to a difference in recording and how that plays with the U9.


Where the U2 and U4 had a prominent bass, the U9 seem to take things a little easier without pushing the bass too far into the background. The bass is tight, precise, has great texture and hits with a healthy dose of impact, but is never overstated. It is exciting and that is very much what the U9 need to avoid becoming dry. The bass sets the rhythm and gets your feet tapping. The kick drum in the Rolling Stone’s Hate To See You Go is a great example of this. At first listen with the U9 I did not expect them to be suitable for the Stones and yet the U9 got my bum shaking with ease. The bass is not just impactful, it has great texture and feels pretty fast and articulate for a dynamic driver. It works great with fast drums like in the Foo Fighters’ Cheer Up, Boys (Your Make Up Is Running), which makes the U9 surprisingly musical.

I did try the U9 briefly without the faceplate and was quite surprised by the change, which instead of extending the bass made the U9 sound thinner instead. So while I previously questioned whether perhaps the Stealth damping technology did not work optimally, it does appear to do just that for the U9. The bass of the U9 is a very good quality bass that is capable of digging deep and hitting hard when it needs to, but avoids becoming to dominant.


The mids of the U9 are excellent and the reason I like the U9 for classical music. Instruments sound accurate and natural without becoming very full sounding. Now, I do like full sounding instruments, but it is a bit of a disadvantage with orchestral music and the U9 do very well in separating the different instruments. Layering is pretty good and the way the U9 reproduce the emotions of different classical pieces is even very good. While casually listening as I was doing something else, I often found myself getting increasingly distracted by the music, which was pulling me in and enticing me to close my eyes and drift away.

Vocals are perhaps not quite the U9’s main strength. The balance between male and female vocals is not bad, although there is a slight preference for female over male. Moreover, vocals lack some definition. Like I indicated with the U2, I am quite spoiled here and so I have set the bar very high, which I think is fairer in the case of the U9 as these are Stealth Sonics most expensive offering. Again I suspect a slight upper-mid or lower treble lift I am noticing here. Nothing too bad mind and simply a consequence of a tuning choice, but if you love vocals as much as I do, I would suggest looking at alternatives. Nonetheless, vocals as such are nice and clear, which can be said for the mids in general as well. Very nice and clear to ensure nothing can escape your attention.


Where the U2 might have been a bit bright in the treble and the U4 a little attenuated, I feel the treble of the U9 is just about right. It is sparkly and yet maintains smoothness. So when I listen to Tchaikovsky’s Nutcracker with the U9 there is a nice sparkle, but not too much, and a slight hint of sweetness to keep things smooth, but not too much. A female soprano voice also comes through very clear and clean with the natural brightness that was missing with the U4.

Cymbals sit well within the image and add sparkle op top, but remain in the background. Perhaps some people like a little more treble prominence, but I think this way the signature avoids becoming fatiguing despite the overall focus on clarity and detail. I have had the Rhapsodio Galaxy V2 over for a while, which also had a focus on clarity and details, and those were not for the faint of heart (and a dream for treble heads). I could listen to them, but would get fatigued after a short while. Not so with the U9. The U9 balance everything out very well and unless the bass would get a boost as well, I suspect any additional treble lift would ruin this signature.



Stealth Sonics have come up with a very nice trio of universal IEMs. The U2 have a signature that is engaging and fun, and I am disappointed I did not have the Final E5000 around anymore because it I suspect the U2 would give those a fair bit of competition. The U4 feel to me like great quality stage monitors that offer the complete package, a smooth and fatigue free signature combined with a very light weight and all the advantages of the Klarity Valve to make them as comfortable as it gets for long, very long listening sessions. The U9 offer clarity and detail at a very high level, while maintaining smoothness and accuracy in the reproduction of instruments. Where the U4 feel like stage monitors, the U9 might lean more towards studio monitors, although without foregoing musicality.

Stealth Sonics offer all their IEMs with a healthy selection of accessories and pack their IEMs with unique technology and styling to set them apart from the masses. At their respective price points the U2, U4 and U9 are interesting propositions, especially for those who prioritize practical aspects such as durability, comfort and a fatigue free listening experience. I feel Stealth Sonics have done an excellent job to create such a complete package and look forward to what they will do in the future.

3 thoughts on “Stealth Sonics U2, U4 and U9

    1. Thanks! I have not heard the Cupid myself yet and the Stealth Sonics review was part of a tour, so I don’t have them around anymore, otherwise I could have begged Alex (Twister6) to borrow the Cupid. Perhaps Jackpot77 (who organised the tour) over at Audio Primate knows how they compare because I know he has the Cupid as well and I sent the tour kit back to him.


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