Pros: Great timber, detailed and neutral mids, best separation of any IEM I've heard; very textured, well-imaged, three-dimensional sound
Cons: Soundstage not as big as TF10, SM3, etc. A bit dark and thick with stock cable
My setup for this review is Foobar2000 + WASAPI Event Out -> HRT MicroStreamer -> UM3X RC with Moon Audio Silver Dragon
I traded for the UM3X about two weeks ago and I haven't used another IEM since.
Currently listening to Classic Sinatra II in FLAC. Everything is well separated, to the point that it feels about as well-differentiated as my HD598 and DT770 in terms of instruments being presented distinctly from one another and not bleeding into each others' space; obviously soundstage size is nothing to speak of by comparison, and the UM3X is on the smaller side as far as triple drivers go, but I think that makes its ability to separate sounds amost even more impressive. Brass, strings and vocals all have appropriate body without sounding overly throaty or bloated, and of course never bleed into each other. Frank's voice really sounds like it's coming straight from one of those old-school microphones. This is a real pleasure.
Vivaldi's Four Seasons with Joshua Bellas first chair, also in FLAC, is next. The bass and cello notes have much more authority and power than I remember the SM3 ever having, for sure; you get that real FEELING of the instrument, whereas the SM3's more delicate sound was more about listening to the bass. This sounds much more "live" in terms of the textures and immediacy of the sound. Those rapid viola lines in Spring are not quite as crisp and distinct as I remember the SM3, but they still sound more "real," with better accuracy and timber. Violins also don't have the same exaggerated feel; they really do peak sometimes and give you that euphoria, but it's not exaggerated. The lack of the SM3's color is really appreciable here.
Now listening to Beethoven's Bagatelle in A Minor in FLAC. Pianos are soft and subtle, but not incredibly crisp here. A very relaxing listen. Everything sounds fine. I've heard phones that definitely have a more atmospheric effect with a piano, which is really important when you're only working with one instruments IMO. There are no SERIOUS faults here, but it's not the UM3X's strong suit.
Bassnectar, however, is. Listening to Underground Communication in FLAC, and the bass is just right here: enough oomph to let you feel it, but never overshadowing other sounds, of course. The UM3X is great for giving bass-centric music plenty of power, but not overshadowing other elements of the music, which is a quality I sought for a long, long time; the SM3 served decently in this regard, but just never felt quite powerful enough; it was punchier than the UM3X, but didn't have the same sense of realistic decay and reverb. Treble roll-off shows a little bit, but not enough to detract from the music IMO. It will never be as sparkly as the TF10, but personally I don't mind, as I prefer a smoother, slightly sound in general. There isn't any information missing, so it sounds good to me.
More EDM: Juno Reactor's Pistolero in FLAC sounds great as well. Again there could be a smidge more treble energy, but I think that's being picky; those who prefer a brighter sound might disagree. Strings have good pluck and the UM3X keeps up with everything admirably, never missing a beat, and giving that psytrance bass punch just enough oomph without bloating it or sounding artificially tight; some people prefer bass that is tighter or punchier than this for faster-paced, more intricate EDM like Juno, but I think the UM3X's more full-bodied low end, which has longer than average decay for a BA, sounds like a more refined, better controlled middle ground between a "technical" sound and what you actually hear when you witness live EDM. You can't feel it thump in your chest; this is an IEM after all, but it's tactile enough that it does justice to the overall sound. And those female vocals...the UM3X makes you feel like you're right there on the stage. Loving it.
Anyone who says the UM3X is lacking in soundstage needs to listen to Mozar's Die Zauberflote (The Magic Flute) Scene 2 - Eine schreckliche Nacht! in FLAC or better, through a decent source, and tell me that distant rolling thunder doesn't catch you just a little off guard. This was the first combination I used to test my HRT MicroStreamer (which synergizes very well with the UM3X for anyone interested) and I literally thought there was a thunderstorm outside! Listening to excerpts from the rest of the opera, the UM3X obviously excels with vocals, and provides proper imaging to give a sense of the on-stage interaction, though not with quite the same drama as some phones with bigger soundstages.
I'm coming to the UM3X from the SM3 and TF10, and although I occasionally miss the SM3's speed and soundstage, the UM3x gives a less colored, more tonally accurate sound that I see as hearing the music rather than the earphone, which is something I'm coming to appreciate more and more as my taste in headphones has gradually changed from bassy/v-shaped towards neutral/balanced and slightly midcentric. The UM3X also never distorts, which was a problem I had frequently with the SM3: it would sound thin and a bit weak at higher volumes: its dynamic sound would begin to falter, bass would start to sound hollow, and the soundstage would compress when I cranked the volume, never really keeping up with what I wanted for my noisy commutes. I can push the UM3x to the limits of my hearing, and its signature and presentation remain consistent; the bass never gives an inch. And while two weeks with a used set doesn't give me much room to comment on durability, I can say without hesitation that the UM3X is made with infinitely higher quality materials just by holding it; the flimsy shells of the SM3 v2 would flex with a small pinch, whereas the UM3X feels solid as a rock by comparison.
Overall I definitely recommend the UM3X for anyone looking for a largely neutral sound with slightly elevated bass. It does full justice to music emphasizing strings and vocals, and performs well with electronic music if you don't have a need for laser-like treble. As a complete package, it is by far my favorite earphone I've owned to date.