So although I'm relatively new here on head-fi, I've lurked on these forums reading up on various IEMs for over a year and a half. In that year and a half, I've had the pleasure of owning both the SM3 v2 and the UM3X RC. Right off the bat the biggest difference between the UM3X and the SM3 is the soundstage, which is a bit smaller on the UM3X (see diagram here). That said the UM3X has very forward and clear mids, albeit dry compared to the SM3. The SM64's soundstage is similar to the SM3, but is a little different and hard to describe exactly. The treble on the UM3X is ever so slightly rolled off, giving a fatigue free listening or monitoring experience (depending on what you use it for ;-) ). The mids on the UM3X are very much forward. Sometimes with some songs I feel like I am the lead singer in the song, which I guess for some can be very uncomfortable from a soundstage point of view. Instrument separation on the UM3X is supreme though, if you love that kind of thing. The SM3 or the SM64 for that matter doesn't even touch the UM3X in that department, and there's a reason I call the UM3X "the shredder".
Alright so down to the SM64 and the UM3X, which I've had the chance to compare recently. The UM3X on its own still suffers the same smaller soundstage compared to the SM64, however the realization of the full audio potential of either one of these IEMs is dependent upon the source dac/amp combination. Straight out of headphone out on a laptop for example, the UM3X will be a little better than the SM64. As soon as you add an amp into the mix the SM64 will clean up with refined treble energy and slightly forward but still lush mids. Thus the SM64 is almost like a sound refined SM3, but with the added treble energy you might find it to be slightly sibilant as well. Sibilance was a slight issue for me, which is why I still prefer the sound timbre of the SM3 and the UM3X over the SM64.
Now on the other hand, the UM3X with the Silver Dragon cable is an interesting story in and of itself. The smaller soundstage that you get out of the box is significantly widened with the silver dragon cable, as is the sound timbre and response. Everything just sounds cleaner and more responsive, and it definitely gives a stiff competition to the SM64 at almost the same cost if you get the UM3X RC on sale. So while you get a stock cable plus the SM64 in $500, if you get the UM3X on sale you can at least get it plus the excellent Silver Dragon cable also in $500 which evens the playing field. Plus the UM3X is, in my opinion, constructed much better and since you live in the US you'll have no problems with using the warranty either.
That said the SM64 is definitely a wonderful sounding IEM, and if you have the money and don't mind the potential issues with EarSonics if you have build quality issues then its definitely something you might want to try out. However I find that the safest and most satisfying route is the UM3X RC with the Moon Audio Silver Dragon cable, IMHO.
Edited by outmywindow - 3/5/13 at 1:30pm