Apparently people who own the RSA Darkstar like it with the HE-6, haven't tried it though. I haven't found a headphone-specific amp that best serves them. IMO, by the time you do find, you'll wish you got a speaker amp instead given the plethora of options and competitive costs. I know size is an issue for some, but if you want the best, Class A is going to cost you some real estate.
EDIT: You could always try a Gainclone/chip amp too. They are small in size, and parts count is under 20. Not sure about synergy, but there are plenty of them floating around on eBay and DIYAudio.
Edited by brunk - 2/24/14 at 12:59pm