Very interesting replies!
Here's the thing - I've only been feeding them 320kbs MP3s and lossless CD imports. So, the source material is not the issue.
This is a common mistake people seem to make in hi-fi: high bitrate /= well recorded. A 192kbps rip of a Dire Straits album is going to sound better than a FLAC rip of RHCP's Californication no matter how you slice it. I don't know what kind of music you listen to, but quality varies massively with modern recordings.
As for the HD 600s, I wouldn't worry about it for a while. You've got the 595s which are damn good cans for what they are IMO, but the resale value of them is not up to much. So unless you can return them (if you wanted to) then you ought to stick with them for a while and work out what you like from them before picking your upgrade path. Not that you ought to upgrade - you should really just take your cans and run far, far away.