I'll let others answer for W4.
UM3X are not really midcentric, although they would certainly appear so if you audition them right after TF10s. But they certainly don't lack bass and treble, hence the neutral statement, although some people won't appreciate the separation (may be qualified as exagerated when opposed to W4).
UM3X vs FX700? I had both until mid-February. Sold both after I got the SE535 Special Edition. I had FX700 first and enjoyed them fully for 6 months, until I find myself wanting better mids, even at the expense of some bass/treble (which were extremely good on FX700). It was granted with EPH-100 and then UM3X.
Had I obtained a better fit with UM3X (most people do, it seems I'm an exception), I would have kept them instead of buying SE535SE.
So the answer is UM3X, signature is more neutral, and I can't live anymore without excellent separation and solid mids. SE535SE are just more comfortable for me and do provide the same qualities as UM3X (but signature is more midcentric than UM3X so I had to EQ with a light V to go back neutral).