I got them for $350 brand new. The HD 650 was costing only slightly less than this (~$50). Bought both but I liked how the HD 660S2 sounded a little more than the HD 650. So the HD 650 went back.
Using them with a modest old FiiO K5 Pro (AKM, not ESS), and quite liking them. The pads hurt after a few hours though. I am used to much squishier pads.