I can say that 215 has a lot more and punchy bass than the UM1.
People buy UM1 for instrumental or vocal focused music, but bass just isn't it's strength, at all.
The bass in UM1 is solid, but severely lack extension and quantity.
A well ran-in 215 can give really captivating bass, with a good player as a premise, from my point of view.
I use my Cowon J3 with my 215 and it's a great combo. The mids may be a bit retrieved (because of the player), but using the black olive tips can pretty much give sufficient mids. The highs have also better extension thanks to my J3. But most of all, the bass sound is the true spotlight of this pair or earphones.
I also own a 535, and i can say they are two distinct earphones, neither of them can be modified to have a similar sound to the other one.
I would say the smooth and rich nature of the mids remain the same for both earphones. 535 has significantly brighter vocals than the 215, especially female vocal e.g. Avril Lavigne, Taylor Swift etc. 215 is quite behind of 535 in terms of female vocal, but shines when it comes to male vocals. Songs from e.g. Michael Buble will sound more melting from the 215. The bass from 535 somewhat lacks quantity and punchiness. The bass extension is there but can rather be felt than heard, while rich, detailed and solid bass is produced from the 215. The bass from 215 will not dominate over the other frequencies if olive tips are used. The sound stage on both earphones are equally wide, in my opinion, 215 have more depth of the soundstage, but with an undistinguished contour. In comparison, the soundstage in 535 has a clear outline and have slightly more height than the 215.
All of these features may vary depending on the music device you use, and certainly with amp on.
In general, one would choose 215 for their bass and warm & comfy sound base, and 535 for vocals and for listening to a larger variety of songs thanks to it's high analytical ability.
