Stealth Sonics U2, U4 and U9

U2 – The fun one.

  • Drivers: 1 x DD (Low/Mid), 1 x BA (High)
  • Crossover: None
  • Frequency response: 20Hz – 20kHz
  • Sensitivity: 103dB at 1mW
  • Impedance: 16 Ohms at 1kHz
  • THD: <=1% at 1kHz
  • Price: US$249



The U2 have to my ears a mildly U-shaped signature with a solid bass that is balanced by a sparkly treble to add a bit of fun and musicality. The stage of the U2 is not the biggest, providing an intimate setting that is complimented by a somewhat holographic feel. It makes the U2 immersive and fun to listen to. While I would say the overall feel of the signature leans towards warmth, the U2 manage enough air to provide good separation and I did not notice much in the way of congestion, even with layered choral music. The background though is not the blackest I have come across. Perhaps I am spoiled, but the image lacks some crispness and definition. Not much of a problem though because the signature is musical enough that it will compel you to tap your feet nonetheless, making the U2 really good fun to listen to.


The bass of the U2 surprised me a little bit because it dug deep, was impactful and tight and yet I missed something I could not quite put my finger on. I suspected that this might have to do with the Stealth damping that Stealth Sonics incorporate in the faceplate and so I did a quick test without the faceplates, which immediately felt like the bass extended a bit better and resonated more naturally. I did not have enough time to do a thorough comparison, but it might be interesting to try out for anyone who enjoys an even deeper, lusher bass. I thought it worked quite well for the Rolling Stones, putting a bit more emphasis on the kick drum and bass guitar, but that might not always be what I aim for. With the faceplate on, the bass becomes tighter and more impactful, something that worked better for EDM. Although I will admit that my inner bass head always enjoys a deeper, lusher bass, even when it is strictly speaking not ideal. Of course you can have both in this case due to the U2’s option to remove the faceplate or leave it on. It is however fiddly to do and I would prefer a simple switch (if it were at all possible with this particular system).


The mids are pretty decent. They are not the most natural sounding, but I will immediately state that as a mid-lover or ‘mid-o-phile’, I am very picky when it comes to my mids. The U2 sound far from artificial or thin, but lack some fullness and definition. Perhaps the U2 have a little too much emphasis on the upper-mids or lower treble, which is particularly noticeable through a lack of vocal density. In choral pieces such as Bach’s Magnificat I find that male vocals lack some of the power they need to balance out the female vocals, but female vocals too lack some definition. Agnes Obel just does not sound like I expected based on the intimate setting the U2 produce. I might need to emphasize here that I do not have other IEMs in this price range for a comparison, so perhaps my bar is set to high and I am just a spoiled little brat because of the much more expensive, mid-centric IEMs I am used to.

Other than that, the mids are nice and clear and even with classical music, which does not play to the U2’s strength, I find that instrument tonality comes through very nicely. Move to proper fun music such as rock and the U2 perform really well.


The treble is a hint towards bright in order to provide the sparkle that compliments the fullness of the bass really well. It adds a little bite to guitar strings and indeed electric guitars sound pretty good. The sparkle and bite are not overly prominent and I don’t think will give many issues to even the more treble sensitive souls among us. (Guilty as charged.) I always like to risk my life for my readers by testing for sibilance with a variety of soprano voices that are high enough to shatter my glasses even when I am using IEMs, and it was quite smooth with the likes of Elin Manahan Thomas.

The treble I feel is just about right to provide balance, a little bit of air and some clarity to the signature and it works really quite well.

Page 3 – U4 detailed analysis.
Page 4 – U9 detailed analysis and Conclusions.

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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