S4 as very good bass quality and quantity. mids are also good with decent soundstage.
they sure must be better than RE2( ....have no experience with re2)
and from what i have read, m2 have more balanced sound and better mids and treble than s4.
BUT why ...M2 or RE2, now that you have the taste of s4, move on to better ones not comparable ones(...IF you were thinking for an upgrade)
better ones would be radius DDM for bass, DBA-02for balanced sound, RE252 for soundstage and balanced overall sound, TF10, umx3,westone 3 , IE8(for these, naam hi kaafi hai)