I have discussed this in the HD800 appreciation thread as well as others. I spent most of CanJam trying out approximately 40 amps with these phones. The HD800 balanced are in a different league than single ended. There were very few I felt excelled with this headphone and they are as follows in the order of preference:
1) Donald North Audio Sonett (single ended or balanced version) Tube Amp: $1200 (or $1000 for all CanJam attendees). I only heard the single ended version of this amp and I felt the single ended version was on par with the balanced versions of other amps below, so you can only imagine how good the balanced Sonett must sound. Incredible bass control and smooth extended treble. Very large left to right soundstage. (Hooked up to Naim CD5x or CD5i, not sure which one)
2) Ray Samuels Audio B-52 Tube amp, balanced. Slightly warm presentation. $5500
3) Ray Samuels Audio Apache solid state, balanced. Slightly warm presentation. $2995
4) Woo Audio WA22, tube amp, balanced (but I think I only listened to single ended). $1800. Very nice soundstage left to right and front to back. Excels in the midrange.
5) Woo Audio WA 6 SE tube amp, single ended. Excels in the midrange. $1050
6/7 Tied) Rudistor RPX-300 solid state, balanced (but I only listened single ended. This amp has a very smooth and laid back presentation, very different from the other amps. $2500
6/7 Tied) ALO Audio Amphora, solid state, single ended. Very neutral presentation, no particular area stood out more than any other. $995
I know a lot of people are mentioning the Eddie Current line up. I did not get to hear the Balancing Act, but I did hear the Zana Deux and HD-2 and I did not care for either with the HD800.
All sources were different and yes, that will factor into it, but all were high end sources. I've listened to the same amp with various high-end sources and the differences are not as great as the variations in amps with the HD800. I would say it's split somewhere between a 20%-35% source / 80%-65% amp factor, so long as your source is in the $1,300+ range.