With grey silicone tips and a deep insertion the W3 can get boosted midbass, but the UM3X does not. With UM56 tips the W3 does not have too much midbass for me, nor does the UM3X, and the two sound closer to each other if I use the UM56 tips. For me, the custom silicone tips and vinyl tips alike improve the W3 bass and treble. Tips are important, as is ear positioning - they improve the W3 bass and the treble alike, at least for me, but do not harm the bass and treble in the UM3X. The W3 treble is better with Complys than the Westone silicone tips, but I prefer the UM3X treble with the silicone tips over the Complys. The bass of the W3 with Complys is a bit stronger than the UM3X bass, but I didn't find it to bleed over into the mids.
Bass levels change with seal, tips that give you a better seal will naturally have better bass. Having said that, if the UM tips give you less midbass, it means the seal isn't as good as some universal tips. Treble improvement, I can see that because the tips will have a much specialized insertion depth, but reducing bass is a bad thing,in the end it's just how the IEM is, unfortunately I think it's bass is excessive.
Edited by Inks - 10/19/12 at 1:40pm