If your budget will accommodate things like McIntosh equipment, perhaps it has room for you to consider getting a dedicated headphone amp that you can either pair with a good preamp or even, in the short run, plug into the tape monitor output of your receiver. The tricky part is that HD 800s are supposed to be sort of hard to drive, so I'm not sure what headphone amp to suggest. Schiit (pronounced just like you'd think -- if you go to their website you'll see they have a pronounced sense of humor) offers a range of excellent headphone amps for different applications -- you might call them and ask which one they would recommend for the 800s. I have a pair of the 700s paired with the Schiit Valhalla and the combination works beautifully.
Other possibilities that come immediately to mind include Fosgate's new headphone amp, which is supposed to be exemplary, and Bryston's, which is supposed to be able to handle any load and has outputs for both balanced and single-ended connections.
Others might be able to offer up their specific experience with the 800s and a particular headphone amp.