Sean primarily compares the TH600 to Ultrasone's Signature Pro.
The TH-600 shares similar tonal signatures to the TH-900 that it's a somewhat mild U-shape. I won't really say that one has more bass or treble quantity than the other, nor ones mid more recessed, laid/pulled back from the other. By comparison the Signature Pro has a more prominent mids, slightly less sub bass, and slightly more forward treble.
I found the TH600 to have one of the tightest mid-bass to mid tone transitions I've heard in any headphone to date. My short impressions can be found at the bottom of this thread.
If you have the TH600 please add your thoughts to at Headfi.