If you love treble, RE-400 may not be for you. It's a bit uneven and quite soft/laid back sounding in the highs. I think the treble character may improve if you remove the foams, although that may create unpleasant peaks in the treble response. However, EQ'ing should definitely help - just ask tigon_ridge.
+1 Without a doubt, ER4S has the best treble I ever heard in IEMs.
Edited by Pianist - 6/17/13 at 7:44pm