I own both pairs of the HD668 that Amir reviewed. One is still new in box, the other I've used regularly for about 3 months. I've compared the HD668 against Sennheiser 58x, AKG 371, Beyer HD880, and Beyer HD770. I have no experience with AKG 240 or Superlux HD681 EVO.
Aside from comfort issues...