Check into the Headroom Micro Amp. It's not a portable and is quite powerful. I bought it mainly due to how it had good synergy with BOTH my HD-650 and AKG (K702/K601) headphones.
It's probably the best amp I've heard for the K702 (under $500!!) and even drives my K601 and K501 extremely well.
I always loved my Headroom Total Airhead, so I figured it could hurt to get an upgrade from my Airhead and Asgard. Whatever you do, don't get the Asgard for the K701. Check first to be 100% it will drive them better than before, but I have my doubts.
My K701/K601 were absolutely awful from the Asgard. Couldn't even drive them well. It seems better for the D2000 and HD-600.
The Matrix M-Stage is also said to be good for the K701, but I haven't heard it.
Don't make a mistake and get a portable amp for the K701. E9 would be a bare minimum.