I have been working with the folks at BitPerfect and they have spent some time examining the high hiss problem that occurs on some 24/192 KHz tracks being fed to the HDVD800. They too have found the problem to be a "really weird" one. In summary, I was told by BitPerfect Support that:
"I am convinced the problem lies within the DAC.
"I will chase after Sennheiser and ask for a unit to test. I have not dealt with them before, so I don't know how responsive they will be"
They sent me another note a little later stating:
"I have raised this with Sennheiser Canada, and the manager there seems to be interested in taking this up with HQ in Germany. So we'll see what happens"
Anyway hope that they can determine that I'm not totally loco here