If you like monitor-type headphones, the KRK KNS8400 offers superior resolution to the M50 at a similar price point. It also bests the M50 in the comfort field, and comes from a company with a much more responsive service department than A-T's. The Sony MDR-ZX700 is also comparable in sound from what I remember of it, but I haven't spent as much time with it as I should to give a full comparison.
If you want a fun-sounding and bassy headphones, there are a large number of options out there that equal or best the M50. The M-Audio Q40 is bassier than the M50 with similar build quality, and while is doesn't have quite the detail of the M50 it tends to be sold for slightly less than the A-T at any given time, and its bass reaches a little deeper than the ATH-M50's does as well as having more punch. Models comparable to the M50 with a V-signature include Ultrasone's HFI580 and HFI780. The HFI580 is somewhere between the M50 and Q40 in sound signature and detail retrieval, and the HFI780 has both more bass and more detail than the M50. At a bit higher price point, the Beyerdynamic DT770 beats out the Audio-Technica in detail, bass slam, deep FR, and even durability.
Nice review I'm sure this will get some people off of a dead set m50 opinion for a first nice can which is good. I had the m50 and loved it but it really overshadows a lot of great cans that are better for some people although I am seeing they are starting to get a little more well deserved light. I am curious to your claim of the dt770 being more durable. My m50 went through absolute hell and were fine. I sold them because they are more of a dj studio beater can rather than an audiophile one. They sound good though
Edited by Frosty3258 - 9/15/13 at 8:32pm