I currently own a pair of AKG K550s, but after a little browsing, I found the DT 660s going for around $80 here in NY. Reading over through several reviews which were pretty glowing about the 660s, especially in the classical area, they seem like a pretty good option for me at a bargain price.
 
How are the DT 660s, and how do they fare up against the K550s? Would they be an upgrade or a downgrade? How is their sound isolation, resolution, and soundstage? Thanks.