It could be sources with recessed mids. The M50 does a have slight dip in the upper mids affecting the tone of female vocals and some strings but the mids as a whole are not recessed and the measurements bear this out. The mids are largely neutral with a slight bass and treble emphasis making a U- shape. This is not the same as recessed mids where bass and treble are neutral and the mids get sucked out. If you use boosted extremes as your reference than you can mistakenly call the mids recessed, but the M50 bass and treble is boosted over neutral where most of the mids are except for that upper mid dip I mentioned.
Along w/ sources having emphasized extremes, different listening volumes will accentuate different things for different people. I've come to know and look out for impressions from folks I know/believe listen at very low volumes as I know they won't correspond to my own. Suffice to say, I never listened to the M50 at low volumes.
You can compare the M50 to the Sony V6 here:
The SA5000 never seemed mid recessed either, I've never run across a single impression of the kind:
The complaint is always recessed bass and maybe some treble peaks at high volume. This illustrates many people are talking in relative terms not in absolute terms.
If the Sony V6 is your standard then the T1, HD800, LCD2/3, Stax and Baby Orpheus are mid recessed too.
-Edit - Plus you might want to give the M50 a week or two just in case they surprise you all of a sudden. You never know. Though the OP's sig preference sounds more like Shure 840 or Senn 558/598.
Edited by Anaxilus - 3/27/12 at 3:21pm