Balanced cable from HD-800 to pigtail 4-pin XLR connector on speaker taps. Put a 10ohm/10watt resistor in parallel on speaker taps with the pigtail. Use a passive or regular preamp to control gain of your source.
Better yet, go to the speaker amp thread where you can read the discussions of folks who got me to try the gambit of speaker amps for HPs and give good advice. The first part of the thread is the most informative. Member "Happy Camper" has also given excellent advice on various threads regarding speaker amps.
Edited by Mambosenior - 11/20/13 at 10:51pm