Hehe, It's funny. The 660 is a clone of beyer headphones and the 681/668b are clones of AKG. The way the pads go on is the difference I think.
The pads on the DT770 "latch" onto a rim while the AKG pads use tension to stay on the rim. Plus the beyers are a liiiitle bit smaller.
Edited by Supertoaster - 3/3/13 at 6:29pm