I quite liked the ATH-M50, but have enjoyed a few over it since. I would ask if there is anything about the ATH-M50 that you would like to change. More or less upper bass? Because the M50's can be a bit boomy, their treble is probably the more lacking aspect though, but 'fixing' these things will fundamentally change the sound and you might not like that. Saying that, your music tastes are fairly close to mine, so here are a couple of recommendations:
SoundMAGIC HP100 (flat response, less bass heavy that the M50, but also smoother and better, much better treble)
SoundMAGIC HP200 (rather v-shaped response, treble can be high)
Denon AH-D600 (if you want more warmth and bass)
Sennheiser HD600 (super flat, very detailed, amps really well)
Senheiser HD650 (like the HD600 but warmer, not too crazy)
B&O H6 (light, comfy, flat like the HD600, but perhaps not quite as exciting)
B&W P7 (warm, lush, exciting, nice mids and treble too)