I think the prob here is that it seems there's 2 different versions of M50 floating around and one is bassheavy like in the graph posted here while the other isn't and has bass and mids pretty much on the same level. (the version I tested, so was like where's the bass boost people are speaking off ^^)
Another thing that speaks for that is how Tyll's measurement of M50 used to show no bass boost whatsoever, in fact it gently rolled off in the lower frequencies but never really elevated higher than the midrange but recently that measurement has been redone and how it looks different with an obvious bass peak, it's about 5~6dB difference in bass between the old and new measurements from my memory.
EDIT: OK the old M50 pdf is still viewable, nice! Take a look for yourselves:
Edited by RPGWiZaRD - 3/13/12 at 5:09am