I've driven mine very successfully with 1) Schiit Modi3/Magni3+ stack, 2) Schiit HEL DAC/Amp, and 3) iFi nano iDSD Black Label portable DAC/Amp. All three rigs have slight differences but all sound great. I was most concerned with the iFi driving the 300 ohm HD6XX but I'm running the volume about 1:00 at good listening level so no problem - impressive little device.
I have a Schiit Asgard3/4490 on the way as well...
A lot of folks really seem to like an OTL tube amp for these phones though. I can definitely see a Bottlehead Crack or something similar in my future.