You will find that HD650's are considerably brighter than HD558's, they have much more treble, very sparkly, but not piercing, and much more detail. Mids are about the same, bass is about the same in most cases, even though HD650's are able to produce considerably more of it.
The thing is though, HD650's are much less fun to listen to than HD558's, IMHO. Today, a year and a half after I bought them, my HD558's still make me go WOW sometimes, HD650's never ever did, not in a single song, no matter which volume I listen at or what kind of mood I'm in. I can feel ready for a party, once I start listening to 650's, serious face comes on and all the fun is gone. Thats my only complaint about HD650's. They aren't dark, they aren't muffled, they aren't slow, they are in fact opposite of all that (at least new models), but it just isn't a headphone that will make you tap your feet or do an air guitar while jumping around in your room until you go too far away from the amp, cable unplugs and speakers automatically start to scream at high volume in the middle of the night...