...yes, I did. And actually the synergy is great. I have never been a big fan of the HD800, but in combination with the HDVD I actually quite like it.
I came to the conclusion, that the HDVD800 was kind of designed "around" the HD800: The damping factor respectively the output impendance seems optimized for the HD800: 300 Ohm / approx. 40 Ohm = Damping factor between 7 and 8. Some claim that around 8 is the optimum for the damping factor...
And here lays also my problem with the HDVD... most of my phones are rather low in impendance. My AH-D7100 with 25 Ohm and the LCD-X with 22 Ohm... both leading to a damping factor smaller than 1. Not good. That's why the sound seems to kind of sticking to the membranes, not really transparent, quite limited space.
Took me some time to understand and accept that, and only after I tried numerous other amps with the mentioned phones I recognized how huge the shortcomings were when combined with the HDVD800.
But again, as you say, with the HD800, especially driven balanced, its a perfect match. Also with the high impendance K-1000 it sounds great, and used as a DAC it sounds fabulous via my speakers (... all inputs but not the USB, with this one I have issues with the configuration, even after the firmware update)...