I recommend the CEC HD53N balanced amp. It cost $1000 and is a very good value. I use it with the HD800 balanced. Balanced is the way to go. Here is my impression of the CEC HD53N.
I am a relatively inexperienced user, but the HD800/Nuforce HDP combo is an outstanding experience.
I am sure it could be improved, and probably substantially, but ignorance is bliss, and I have found this combo to meet my every expectation with any type of music.
If I had never heard the HD800 on a $2500 ZDT amp, and no other rigs were available, I'd still be happy to listen to them on the HDP all the time, just not with the stock cable.
I've been using my HD800 with an RSA Protector balance out and a VDac as source. Im having such a great time with it the Beta22 (3-channel active ground) is gathering dust for now. Cardas cable upgrade for $8.0/ft is an excellent addition as well.