I have owned the UM3X before. Unfortunately, one of the drivers broke and since I live in the Philippines, it's rather hard to get the warranty. Eventually, I had to buy a new pair but decided to go for the TDK BA200. The main differences in the sound would be that the UM3X has slightly less bass, more forward mids, more accurate highs and much better separation. Don't get me wrong, the BA200 are great, and they do sound quite similar. However, comparing it to UM3X(and i'm being critical here), the westones win. As for your preference with the bass, I wouldn't recommend the UM3X so if the lower end is your priority, you can cross that off your list.
I think the um3x have slightly more bass or at least equal bass quantity to the ba200. The um3x is slightly more forward in the mids than the ba200 is. The ba200, although still warm and smooth, sounds a little more neutral (and slightly thinner) overall. The um3x is indeed an upgrade.