Good question. More than likely the Valhalla 2 or the Lyr 2. I need to do some more research and see which would be the better fit. I'm already indecisive enough about a DAC, let alone a new amp.
I may have a bigger issue though, as I am a very new HD800 owner and am still adjusting to the sound, come up from the HD600. I do have the SDR mod installed, but in one ear things are often still too bright.
Regardless, I don't see myself deviating from the Schiit brand anytime soon. I'm not in a position where I can sample and return things over and over until I find something totally perfect, but Schiit comes pretty damn close and threads like this help me know what I'm getting into.