if you are listening to a really overly bass heavy mix, like tons of mixes i listen to are, then yea it will be a headache and not nice. but if you listen to well mixed music, it sounds really good. but seriously, you can really crank these and there is like no distortion at all....so be careful you aren't listening too hot for too long or headache.
im tempted to mix with them because the bass sounds so good, but i wont lol. thats backwards if you think youre going to mix with these, they arent mixing headphones. you mix on something flat, and you make sure you dont push it too hot, and then when you listen on these after, it will sound good. if you push your mix too hot on your mixing heaphones, then it will sound bad on these.
mixing with speaker monitors though, always is the great benchmark. you mix with headphones for an hour, then mix with speakers, and you find the equilibrium. these m50x, you do not mix with them. they are for post-mixing, and maybe well if you dont understand your mixing headphones gainstage then you could use them to actually reference how overly hot you are mixing with your mix headphones.