if you like hip hop or anything bassy, you likely will not be happy with a bass light set of cans. classical however would likely sound good one them.
if you live in the usa, ignore the m50x as its overpriced at $170 or whatever they are charging for it now. you can get $99 open box but brand new m50's or $120 sealed box from most places. for that price they are a good deal, but for above that the dt770 is better.
i personally own an m50 and have for at least 4 years so i can tell you personally what i think of them. i listen to dubstep, electronica, jpop/kpop, rock, alternative (and basically everything except country, most rap and very minimal classical). sound quality is good for the $100-120 range, they are certainly pretty bassy without getting muddy treble but not so bassy as to be in the realm of bass cannons. they are built like tanks and besides the earpads they look brand new. even the cable is long and thick with a metal connector end. i've found them comfortable (but not for people with glasses or very large ears) but like all closed cans they do get a bit hot (all closed cans do). they have a v-shaped sound curve which means recessed mids and more prominent treble and bass, also known as the fun-sounding shape. they do have a very small soundstage (which means less airy sound, more direct.... in other words its more like being in front of a band at a bar instead of listening to a band in a park). overall for my needs they have worked very good and arent a bad choice for the budget if you realize their limitations, how they will sound and dont believe the crazy hype about them (they are good, but not a miracle set of headphones).
the hd558 are more neutral though tend a little towards bass compared with its big brother the hd598 which is the most neutral. generally the hd518 hd558 and hd598 can be compared in that from low to high they get less bassy and more neutral and from low to high gain in soundstage and quality of sound. the m50 is significantly more bassy than the sennheisers because its capable of sub-bass (the really low hitting bass used in dubstep and similar) just like the dt770 are. many cans do not hit this low and only handle mid-bass well.
just how bassy of a can do you want, what are you expecting them to sound like? a little more information is needed.