Everyone should get an amp when buying the K702. The Total Bithead previously mentioned cant drive the K701/K702 well. It's basically a $99 Airhead with a DAC built in. It's mostly a portable amp. It can drive the HD-650 quite well, but had trouble with my K701.
The E9 is a great starter amp for the K702, but it doesn't make the K702 sing. I upgraded to the Headroom Micro amp and it really makes them sound so good! For $350 it's quite powerful and even more powerful than my E9 and Asgard. It's half the size of the E9 too and is built like a tank!
E9 will be fine, but a better amp with good synergy will take it to a whole new level I think. I also heard the Matrix M-stage is quite good for them.