I had returned 650 and got another 600.
Just the opposite of my GF, she originally had a HD-600 and returned it for a HD-650. That's why Sennheiser will keep making both, some like the HD-600 others like the HD-650. While I prefer the HD-650 sound signature, the HD-600 is its equal with a different sound signature, neither is better than the other, its only personal preference as to which an individual will like.
I don't understand those who say one or the other is "junk", its just a personal thing that one prefers over the other.