I would be pretty easily convinced that there are two different types of m50s (Sound-wise that is. i dont mean the cable.) based on direct comparisons between mine and a friend's. I figured they would be pretty similar, both being the corded version, but they were substantially different.
The first thing I compared was bass. Mine have a very deep, sub-bass emphasis, but only when the song calls for it. Otherwise it's generally present, noticeable, but almost never overwhelming. His have earth-shaking bass. Literally. When the bass hit, I took them off because for a split second, I actually thought the world around me was shaking, and that is not a compliment. The sub-bass was still there, but with a ridiculous amount of muddy, bloated mid-bass added.
The mids are noticeably recessed in mine, which is unfortunate because they sound great. They have a very smooth, natural sound, but I consciously wish they were more forward while listening to them. Some songs just sound hollow. But if mine are noticeably recessed, his are ridiculously.
Mine have clear, extended highs that are sometimes pretty sibilant with low-quality music. His will never, ever be sibilant. The high end is practically non-existent, which is fine for him because they have teeth-rattling bass, and because he's an idiot.
Overall, his pair is like listening to a rap concert through a brick wall.
It seems to me that either there are at least two different kinds, or some just randomly sound different from others.
m50s are still a great set, and they blow hd25's out of the water. Those are getting traded for some srh840's. Hopefully these prove to be better competition, because they are both considerably higher priced.