I'm using a Headsave Classic. I tried them with my Perreaux SHX-1 0.5, and it didn't work well at all. It sounded quite bassy and lacked detail. I may be looking for another amp at some point, because it seems like the K701s are more sensitive to differences in amplifiers than my Sennheiser HD600s.
Woo Audio 3. I'd heard that the 701's needed a lot of power, but my Woo has no problem driving them, and they also played very loudly with no sense of strain when run through my AV reciever and a lowly boombox.