Pros: Good separation, pleasing midrange. Airy and engaging
Cons: Highs roll off
I must say the UM2 sounds better than the UM3x which I sold shortly after few months. In short; it sounds more articulate,airy and more expansive. The UM3x sounded too bloated,closed in and collapsed to me; the soundstage was too narrow and the UM2 doesn't share the same trait which is great. The mids were too forward and the bass was too much, the highs are too soft and delicate on the UM3x; it didn't have the balance the Westone 2 does. UM2 sounds quite different from the UM3x and I can't see how anyone can say the UM3x is a definite upgrade from the UM2 in sound other than having one extra driver which doesn't mean much. The Westone 2 is probably the IEM I've kept longest, longer than the Westone 3 and the UM3x; I am quite fond of it. If I had to rate all the Westone IEMs I owned it would be UM2 >= Westone 2 > UM3x >= Westone 3 >>>>UM1
Note that I have seen two versions of the UM2, whether they sound different I have no idea.
The first revision has clear stress relief strains with orange comply tips while the latest ones have black stress relief strains with gray comply tips.
Shell size: The UM2 and UM3x share the same shell I believe and it is noticeably bulkier than the Westone 2; I could lay down on the bed with the Westone 2 and not feel a thing and I can't say the same with the UM2. If I had to estimate, I would say the UM2 is at least 33% bigger than the Westone 2. It is quite a bit thicker than the Westone 2. The glossy finish on the Westone 2 is more prone to scratches than the matte texture finish on the UM2 shell. Comfort I'd say the Westone 2 is definitely better, just plain smaller and lighter.
Cable: The Westone 2 and UM3x both uses braided cables but the Westone 2 is more tightly braided while the UM2 is not so. The cable on the UM2 is definitely softer and flexible than the Westone 2. If I coil up the cable on the Westone 2, it is harder to straighten compared to the UM2 meaning it likes to retain its previous shape. The plug on the two are different; UM2 being bulkier and Westone 2 being thin to fit on the devices where the 3.5mm socket is recessed.
I find the sound didn't change much at all using different silicon tips so I stuck with medium sized gray silicon tips
Highs: The extremes of the Westone 2 extend better than the UM2. The highs on the Westone 2 is more pleasant to listen to; it is quite safe sounding and is never offensive. The biggest difference I notice is that the highs are more articulate on the UM2 and I hear some low treble boost on the UM2 which makes snare drum hits and rolls more lively. Cymbals sound more lively on the UM2 and triangle sounds sparkly on the UM2. The cymbals also sound more splashy, which can be fatiguing on the UM2 compared to the Westone 2. I have no real complaints about the somewhat common feedback of roll of highs on the UM2 but then again I do not listen to classical.
Mids: Vocals on the UM2 sound thinner and doesn't have the warmth of the Westone 2. However the vocals on the UM2 sound more real and live; it is more revealing. I notice that sometimes the UM2 can sound sibilant on the S's from the singer's voice which is a surprise; the Westone 2 does not sound sibilant at all. Sibilance is the biggest problem I have with the UM2 right now on specific tracks and as far as I know; armatures do not require burn-in. Vocals are more dry sounding the UM2. The midrange on the Westone 2 has a certain smoothness and warmth over it and it is more forward sounding than the UM2. The midrange on the UM2 does not sound warm, it is on the neutral side and it sounds more transparent. Guitars sound more crunchy on the UM2 than the Westone 2. This part is definitely a preference thing, some may like a warm midrange which the Westone 2 but the UM2 resembles my Grado SR325 a little more which is a good thing for me though I still think my vintage SR325 is simply way better in the midrange than any IEM I've owned but that's a different story. Too much warmth to me means it doesn't sound live anymore.
Low: Definitely more bass on the UM2 and on the lower extreme as well; not boomy and quite controlled. The speed is quite similar. Because of the bass presence, the UM2 sounds bigger than the Westone 2. It doesn't mean the Westone 2 has no bass; it's there but not as loud as the UM2. The UM2 can slam while the Westone 2 can hit. The UM2 does not have as much bass as the UM3x that's for sure but the sound is definitely more balanced. From the day I had the Westone 2, I really did feel it could use a bit more bass and the UM2 does it right.
Soundstage: The soundstage on the Westone 2 is slightly wider than the UM2 but the depth of the UM2 is deeper than the Westone 2.
Detail/Separation: There is noticeably more detail on the UM2 than the Westone 2. Everything sounds more blended in on the Westone 2. On the UM2, the details in the background jump out more and plucking on an acoustic for example, is noticeably more distinctive and edgy. UM2 is more analytical and is more capable of exposing flaws than the Westone 2. Westone 2 is more forgiving, and just makes everything sound great.
The UM2 sounds bright compared to the Westone 2 which is a surprise. I almost felt fatigued after hearing the UM2 for the first time but my ears adjusted; somewhere in the upper mid and low highs can sound harsh depending on the song. The Westone 2 has a safe warm sound; works well with everything but doesn't excel at anything in particular. UM2 takes more chances but like a double edge sword; sometimes it's great and there are times it is harsh and fatiguing. Everything sounds more raw and articulate on the UM2; it sounds more open despite having a slightly narrower soundstage than the Westone 2. I still need to use the UM2 more to see if I could get use to the certain harshness or brightness I hear in the 7kHz-10kHz region area based on listening to percussion frequencies from various cymbals but other than that I am liking these so far. It sounds musical to me which is good as these are stage monitors after all.
(11/20) Update with shure black olive foam tips on UM2
I have been using the shure black olive foam tips with the UM2. I notice it will tame any peakiness, give a fuller smoother midrange but the bass impact has suffered without a doubt. There is no more sibilance with the foam tips but sibilance with the silicon tips is only noticeable on bright recordings. Just about every con I mentioned earlier about the UM2 has subsided except the weaker bass has become a new con with these foam tips. The articulate sound and separation remains strong, which is on a different level beyond the Westone 2. To me the better value IEM is the UM2 at $279 vs the Westone 2 at $249; I would pay the extra $30 every time for the separation and non-congested sound.
I've kept this IEM for a very long time. I still get goosebumps in some songs I listen to. I have not been so content with any IEM like I have with the UM2. I know its flaws but the pros of it outweigh the cons for me. I listen to a lot of modern rock and I feel these sound a little bit like Grados in some way. The midrange is forward and the guitars crunch. I have not thought about upgrading at all and I am still listening to it everyday to this day. I have gotten the cable replaced because something was loose on one side so it has the thinner headphone stress relief strain now like the Westone 2. I prefer the older one. I have not been content with the UM3x and Westone 3 but this one IEM is a keeper!