Tyll reviewed the M50x. http://www.innerfidelity.com/content/legend-continues-audio-technica-ath-m50x
On page 2 he said
As you can see from the frequency response plots of the ATH-M50 and ATH-M50x above, the newer model has about 3dB more bass response, and the peak at 10kHz is substantially reduced. The treble brightness of the old headphone can also be seen in the 300Hz square wave responses (at right). It appears the new M50x has a leading edge spike lower in level and closer to what I'd consider appropriate, and the amount of subsequent noise seems reduced at least by half. I'd consider these measured improvements moderate, yet well focussed on the weaknesses of the older headphone.
The 3rd page you can see his own measurements of the M50x which you can compare to the his measurements of the M50 here. http://www.innerfidelity.com/images/AudioTechnicaATHM50B2012.pdf
According to Audio Technica the difference between the cans is only supposed to be the headband and earpadding material and replaceable cables but apparently there is a noticeable difference in sound. Tyll prefers the M50x.