I've had the M1 -M2 -M3 and M4, so if you are looking for a clear sound, stay away from the M4's.
M3 have the more detailed sound of the M series, they also have a very nice lower frequency response, you can wear them all day and not get stressed.
M2 has a good sound quality but their main quality is the bass.
M1 is too bright for my taste, they sound very clear, but they are boring after a while.
M4's are like a boombox, i just don't like them at all, the higher frequencies are almost lost.