Thanks for all the suggestions.
I recently got a used SPL Auditor and it pairs very well with both K501, K601 and an K240df. I never realized K240df sounds so good before mated with auditor.

In addition, I really like its outlook (in black).

I believe O2 and the sunriseII will also be good choices for these AKGs. But frankly I want an amp which looks more robust.
That being said, I am not so sure if Auditor will be good for K701/2 or Q701, since it appears Auditor performs better with high impedance headphones.
Another characteristic of auditor, when compared with corda arietta, is that there is less fatigue when listening music through auditor.