Don't own the RE0 but I do have a pre-production pair of the RE3, which is the HiFiMAN top of the line IEM. To me, the RE3 has a nice, pleasing mid and upper end, but as is the case with the RE lineup, bass is just not enough for me un-EQ'd, and I am not basshead. The two phones are very different, but I would not call the UM3X dark, but powerful, yes. Depends on your sources, I suppose. Mark and I don't share a single player, I don't think. And to me, the UM3X are not at all dark, just right in fact. But different strokes.
As for the comparison with the RE3, if you have a good, strong EQ option, like Clear Bass on the Sony players, then you can get the RE3s to sound awesome. The mids are a little forward, so I knock them back a tick or two on the EQ as well. Then, excellent sound. But without that, to me, the RE3s just sound very thin (on the Zune, for example, which has no EQ, or the Sony/Fuze set to normal).
Now, as for the the W3 and the UM3X, as I said, I would love to have both. But I was one of those folks who could not use a short tip with the W3 because of the horrible sibilance. So I had to use the modded tri-flange or large Comply and EQ the highs (which the larger tips seem to veil). Again, not a match with my Zune, which is my mega-track player right now.
I did like the warm wow sound sig of the W3s, and will miss it. But the combo of comfort (with the Klipsch gels right now) and the fairly neutral sound sig (across the three major areas) is a much more favorable sound to me, moreso than any other IEM I've owned, and I've owned a lot.
Hope that helps.
It came down to getting the UM56 for the W3s and hoping they would be perfect, or sellnig them and trying the UM3X to meet my needs. The latter strategy worked. Though I may buy the UM56s too, at a later date, based on some of the feedback from those who added them to the UM3X mix.