M50x have heavy low end, and stupidly sibilant highs. I can use 668B's that I modded to be open as hell, and are insanely sibilant for about an hour before my ears hurt too much, and the M50x somehow manage to top that. I don't get it. Very few other people seem to have this problem either.
M40x are much closer to neutral. Low end extension is better in my opinion, but that may just be the low end swamp of the M50x making it hard to really hear. Soundstage is better on the M40x too, though neither are particularly wide.
Both have god-awful stock pads. Swap them with HM5 pleather. Velours make the highs even worse on the M50x. Didn't try them with M40x as they weren't available at the time. The pads widen the sound-stage to a reasonable level too.
POSIX23 reacted to Moist_Sock in M40x vs m50x
