I agree with you
I bought a pair of UM3X last month, since then I have been using the UM3X daily. I prefer UM3X to SM3, for its more present high mids, consistent perfomance on all type music, and the friendly Y-split of UM3X ^^
What meurglys0 said seems to make sense to me. The lower midrange bump and the dip in the high mids are, in my opinion the reason of inconsistent performance of SM3. Like you have stated, in some tracks the SM3 does infact can sound too thick and warm in the mids, and I also find that sometimes vocal doesn't sound as forward. So does SM3 actually able to present music in the way they are recorded, without adding any colour, or is it just simply because there is a graph behind to tell her to do so? Just some of my silly thoughts
However when I switch from UM3X to SM3, I appreciate the airiness. I think I should try photofan's mod.
I had similar problem. I sent my SM3 to EarSonics and they found that there was some problem in the drivers. I think you should try other sources to see if the same thing happens, and sent a mail to EarSonics, or the shop you bought your SM3.
Now if only we can find an IEM that has the best of both worlds: SM3s soundstage and a tamer midrange.