It drives them extremely well and sounds good. Even better than my Asgard which doesn't seem to have good synergy with the K702/K601. HD-600 is better on the Asgard though of course (but not the HD-650).
The K601 on my Asgard (only) has this really annoying lower treble peak, but on the E9 the K601's sound signature is quite smooth and very non-fatiguing with MOST of my music.
I've found that the K601 is just very non-forgiving of very brightly mastered recordings though. Specifically some annoying Japanese Pop i have. It seems like maybe the K601 pairs up well with the E9 because it's fairly neutral. That slightly lower treble peak is less of a problem on the E9 with the K601.
I think my K601 sounds better out of my E9 than the K702! For my K702 I prefer my Micro Amp by far. My HD-650 I also don't love on the E9, but it's really quite good on my Micro Amp.
BTW I know volume doesn't mean anything, but with my E9 and K601 I have to crank the volume up about 5% more than the K702, so not too bad.
good to know, If i want to experience the K601 to the max I'd need to get me a good amp.