Currently my K701 is driven by my Schiit Lyr which is know for having (my favourite word here) gobs of power.. ive also heard it drives them really well and with excellent synergy, which additionally encouraged me to consider them. Also the sound signature.. excellent detail and musicality at the same time. Tyll prefers the HE-500 to the HE-6 and HD-800 (i think)
Regarding the K701.. i think that different amps can drive them, but you really need to put effort into finding one. Overall, they are not worth the effort in my opinion, but I got very lucky with the Lyr. I think its probably the best amp for them. I really cant see the fatiguing sound that SS stuff gives... they are actually good even for rock for the first time. If there is any K701 enthusiasts.. I encourage them to try my combo.