I don't have HD600's only the HD650's and the HD800's.
Yep, got the bifrost and then upgraded it later to the uber analog and USB boards. I also got the Loki and Vali. I now drive the HD800's and HE500's with the Asgard 2 serving as pre-amp to my Taboo MKIII. The Vali is also very nice with the HD800 with or without eh Asgard 2 as Pre. Have not done extensive listening with the HE500's yet. I expect to do some over the break after my jergulor pads get here.
I meant HD800's my bad, typo.