Superlux hd668b has same sound signature as that of Beyerdynamic DT 990. However, Superlux hd668b cost 1/8 times that of Beyerdynamic DT 990. So is it worth to buy Beyerdynamic DT 990?
I can't say I've tried the hd668b, but this review here might help: http://www.head-fi.org/products/superlux-hd-668-b
Looks like some pretty positive reviews. For that price, you can't really go wrong.