Not muddy but more veiled sounding than SE535 for sure. If you tend to listen on the loud sound the UE900 will distort a bit as well which I find bothersome, and this is amped or unamped. This just doesn't happen with SE535.
Some will disagree but as a whole package I cannot consider UE900 an upgrade over SE535. Without hesitation I would take SE535 for rock, vocals and jazz.