IMO, my foray into speaker amps is only just starting. I was planning to use the Emotiva mini X for my HE-4 since it need a lot of power (like the HE-6). As a corollary therefore, any good headphone amp like the GSX-mk2, the Mjolnir (that I own), the Japanese-made Bakoon amp etc will be equally good with the HE-500. The planars in general respond very well to more power so speaker amps should drive them well. Head-fiers seem to like vintage Pioneer, Kenwood and Sansui amps.
I have not heard the HD 800. From what I hear, its a detail freak. For my kind of music, I don't need that detail. People have complained about its treble though. Sorry I cannot help you there. But it also depends on the type of SQ you want in a headphone.
Thanks for the opinion. I'm familar with HD800 sound. I read before that HD800 doesn't have good synergy with emotiva speaker tap, it can too bright sometimes. That was I meant to ask. Since GSX-mk2 goes well with both HE-500 and HD800, I'll look into it.