Everything still sounds better, regardless of the recording being great, good, passing, or horrible. I listen to fair amount of early black metal, and that sounds fine. Classic rock actually tends to sound really nice on likes of HD650 because recordings from that period are really midrange centric, and that is where HD650 shines.

>Would buying one of these headphones just to listen to music on spotify be a waste of money?

Nah, Spotify Premium just straight up sounds great. Most people won't be able to tell it from CD rips.