That's something a number of us are interested to know - which reference grade DACs do you refer to?
I'm loving my Bifrost/Lyr combo with my hd600's. There is no listening fatigue whatsoever. In fact, the only negative thing I can come up with in regards to the Bifrost, is that it doesn't have the resolving power of the "reference grade" DACS. Of course, some folks may prefer this as it keeps it from sounding dry and analytical. Still an excellent piece of kit for the money.