proto_keysI went through many amps with my hd650. Finally settled on the schiit Valhalla 2 after many years trying different set ups. It is perfect! True synergy.
proto_keysI don't have these yet (September ship date) but I AM running Beyerdynamic DT880s (600ohm) through my Crack with a Schiit Bifrost DAC. I have an older Bifrost, so non-4490/Multibit, but I still love it. If you have the money it's a great DAC, but its more expensive than the Crack by a good $100 so /shrug.