Sound:
For serious listening with my UE900 I used and still use only source devices with a low output resistance as well as the largest included silicone tips.
Tonality:
Those who hoped for a successor to the Triple.Fi 10 might be a little disappointed with the UE900, because the in-ear is by no means a new edition of the v-shaped-tuned fun in-ear, even if the shells' colour scheme might suggest so; instead is definitely closer to the Ultimate Ears Reference Monitors in terms of tonality, tending towards a neutral direction.
Diffuse-field neutral in bass like an Etymotic ER-4S/ER4SR is what you won't find in the UE900, but instead around 5 dB more quantity in the bass compared to the Ety, which could be seen as an approximation to the "case of the missing 6 dB"/what most people would consider neutral without sterile/lacking. By the way, the level is maintained over the entire bass range with no sub-bass roll-off.
Subjectively, however, the UE900 sounds a little less pronounced (than 5 dB) in the low frequencies the bass is perceived as slightly less prominent than for example the InEar StageDiver SD-2 (that sounds fuller/warmer) and noticeably weaker than the Custom Art Ei.3 (that sounds fuller and with more bass impact as well), which is mainly due to the fact that the midrange-to-bass-transition happens quite high on the UE900, namely around 700 Hz, wherefore a real differentiation between the mild bass lift and the midrange isn't fully possible anymore.
Due to this rather late transition from the fundamentals to the central midrange, one cannot really speak of a pure bass-only accentuation; rather, it seems as if the midrange simply has less presence/proximity compared to the UERM, which is because the UE900's recessed upper midrange/presence range. By the way, compared to the UERM, the UE900 has got ca. 2 dB more bass quantity.
In the presence area/upper mid-range, the level drops evenly, which gives the UE900 a rather dark/inoffensive character in the mid-ange. The lower mids, on the other hand, even if they are a little warm, don't convey the warmth and fullness of other models, as the transition from the fundamental range to the midrange is very late and actually takes place just before the central mids. Fortunately, the UE900 lacks this "bloatedness" and fullness in the midbass and fundamental unlike in-ears such as my Westone W4R or (to a quite high extent) Sennheiser IE 80.
Following the relaxed character of the lower treble/presence range, the middle treble can also said to be on the somewhat more relaxed side, but without being over-smoothly-relaxed as my Shure SE846's or Westone W4R's central treble.
In the upper reble range between 7 and 8 kHz the level returns - even if this peak doesn't cross the zero line here and remains quite inconspicuous even with sine sweeps, it still helps with adding some freshness and brightness that would otherwise be lacking. I would definitely not describe this range as "sharp", but as "direct" and "crisp" as well as subjectively somewhat bright.
Above 10 kHz, the super-high frequency extension is (very) good and you can still perceive subtle "glitter" to a sufficient degree, if the recording is good enough.
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By the way, the brain quickly gets used to the midrange tuning and perceives it as natural, even though a moderate underpresence of the overtones always remains somewhat present, and female voices in particular lack some brilliance and presence, while male voices simply don't appear as detailed as they should due to those relaxed upper mids.
Accordingly, switching directly from the UE900 to many other in-ears is hardly a problem and the brain quickly compensates for the somewhat subtle presence, but owners should avoid switching directly to in-ears that are somewhat accentuated in the presence range, such as the Etymotic ER-4S/ER4SR, because this can quickly lead to an unpleasant facial expression.
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Finally, I can only repeat that I would characterise the UE900 as "easy, neutral to balanced, with moderate warmth and darkness in the midrange".
It sounds harmonious and balanced and never intrusive in the midrange due to the even drop in the presence range.
Nevertheless, I wouldn't recommend it directly for mainly vocal-heavy music, because voices are not its strength. In (instrumental) jazz it also seems a bit dull in the upper midrange.
For me personally, the UE900 is mainly used as a balanced/tendentially neutral in-ear in electronic music, pop and rock, where its moderate midrange weakness is not or not really perceived negatively.
Resolution:
As for the resolution, the UE900 definitely does justice to its price (based on the RRP of $399 and what I paid on Amazon) and presents a precise, detailed and well separated overall picture.
Details are never missing and the UE meets the requirements of a good quad-BA in-ear.
Even though the UE900 uses back-vented drivers in the bass range, its lows are pleasantly tight, controlled and fast, but compared to some very well implemented closed BA woofers slightly softer in attack. It's definitely a bit tighter than my InEar StageDiver SD-2 and especially compared to most in-ears with dynamic drivers, though.
The UE900 has got slightly poorer midrange resolution compared to the bass and treble - even though its absolute resolution in this range surpasses many dual BA in-ears and offers good speech intelligibility, the UE simply reproduces more detail in the lower and higher frequency ranges. The somewhat unfavorable midrange tuning also contributes to this effect.
With more vocal-heavy music and jazz this is more noticeable and this range seems a bit dull, whereas with electronic music and pop the midrange hardly appears to be less well resolved, if at all.
The highs of the UE900 convince with (very) good note separation and remain clean, detailed and well separated even with complex music material. Not much i missing from the flagship class of the in-ears in the range of $1000 and the UE comes pretty close to the treble separation of my Ultimate Ears Reference Monitors, but without their sometimes somewhat sharp emphasis in the upper highs.
Soundstage:
The stage of the UE900 is quite wide and overall somewhat wider than deep. It also already leaves the base of my head/ear.
Instruments and tonal elements are clearly separated from each other and have clearly marked borders. The ability of more spatially authentic in-ears such as the MR3, SD-2, UERM, Fidue SIRIUS, NocturnaL Audio Atlantis, Campfire Audio Andromeda or Shure SE846 to project spatial "emptiness" in a credible way is something the UE lacks, and it overall doesn't sound as three-dimensional or authentic as the other in-ears just mentioned when it comes to soundstage realism. For the price range, however, the spatial presentation is still good.
Compared to some other Multi-BA In-Ears:
Ultimate Ears Triple.Fi 10:
The classic Triple.Fi 10 tonality presents a traditional v-shape tuning with an elevated bass, midbass and upper treble that can be characterised as somewhat metallic and sparkling.
The Triple.Fi 10 therefore has the more present bass and upper high. In terms of super treble extension, the UE900 is somewhat superior to the triple-driver in-ear and presents the more perceptible subtle glitter.
In the midrange, the Triple.Fi 10 appears more distant and cooler, while the UE900 appears fuller in the lower mids.
The UE900 has a slightly higher resolution level - even in the midrange. Bass attacks on the Triple.Fi 10 are somewhat tighter, while in terms of speed and control both in-ears play at eye level.
The stage of the UE900 is a bit wider in my ears, and in contrast to the Triple.Fi 10, which plays quite flat, it has a bit more depth and better instrument separation.
Westone W4R:
The W4R has the stronger bass and a midbass boost, which also affects the fundamental range, making it sound fuller, bassier and warmer in the bass. Depending on how you perceive it, this bass hump can also have a somewhat exhausting effect.
The midrange of the Westone is a bit more present, while the W4R is even more relaxed due to its stronger 5 kHz recession.
In the upper highs, the Westone is more reserved and less bright, with comparably good super treble extension.
In the bass range and midrange, the Westone has a slightly better resolution - I'd say it feels like about half a class. Both in-ears have an equally controlled and fast bass range, while the Westone is a bit tighter impact.
In high frequencies, however, both in-ears appear to have the same resolution.
The stage of the Westone is even a little wider than that of the UE900, but has practically no spatial depth. Here the UE succeeds better in giving a more dynamic soundstage impression. When it comes to instrument separation, however, the W4R has a slight advantage in the end.
Noble Audio SAVANNA:
The UE900 has slightly more bass, but it doesn't roll off towards low bass while the Noble's does. The UE has the slightly coloured midrange and less level in the presence range. The SAVANNA appears more even in the highs and, unlike the UE900, has no peak in the upper highs - here the UE900 appears brighter and "crisper" in comparison. All in all, the Noble has a somewhat more realistic and neutral tonal tuning while rolling off towards the sub-bass.
As far as the general level of detail is concerned, both in-ears have an equally precise, detailed and controlled bass range. In the mid and high frequencies, however, the SAVANNA has the very slightly higher resolution.
The stage of the UE is slightly wider, but has less spatial depth. Both in-ears separate instruments with similar precision, while the UE is inferior to the Noble when it comes to staggering precision.
Fischer Amps FA-4E XB:
The FA-4E XB is the noticeably bassier in-ear and also has the warmer lower midrange. Voices on the UE, on the other hand, appear somewhat darker. In the upper treble, the Fischer Amps in-ear sounds a little brighter and more energetic.
As far as resolution is concerned, I see both in-ears at eye level - the Fischer Amps FA-4E XB is a bit softer in bass than the Logitech/Ultimate Ears UE900 that sounds a bit less detailed in midrange than the FA-4E XB. The treble on the other hand, is level.
The UE has the slightly wider stage, while the FA-4E XB has noticeably more spatial depth, which makes it appear a little more authentic in spatial terms; it's got better layering characteristics as well. In terms of instrument separation though, the UE900 is superior to the FA-4E XB.
Conclusion:
High resolution and precision; wide but rather flat soundstage; heading into a neutral tonal direction with upper mids that are on the darker, more relaxed side.