I am very, very new to quality headphones and just joined head-fi today, so take my comments with a grain of salt. I purchased the HD558, HD598 and HD650 to use with a Xonar STX PC sound card. The 558's were a bit sloppy in the bass but oh so comfortable. The 598's were very detailed with less bass than the 558's. The 650's are perfect. I did notice that they are not as comfortable as the 598's due to the clamp force being a bit higher, but the pads are very soft and help ease the pressure a bit. I think with time they will stretch a bit (from what I have read that is true) but they are by no means uncomfortable. I wear them with glasses and haven't noticed any fatigue after many nights with several hours each night with them. The 650 definitely has more bass than the 598 and the sound is just so lush. Again, I am very new so I do not have the vernacular or experience of others around here but I know what I like and I am very happy with the 650's. I have returned the other two headsets (thank you Amazon return policy).
Also, I picked up the 650's as an open box on amazon and paid less for them than a set of 600's, so deals are out there.