If I was to recommend a Solid State amp for the HE-6s. I would say go as much Class A as possible. Now from what I read Class D amps sound no where as good as Class A amps (Note - this is only what I've read).
However, the norm and what I have at the moment is Class A/B mono blocks from Emotiva (2 X UPA-1s). Doe they sound good - Heck yes. Better than any other Headphone amp I've heard with the HE-6s.
Now Pure Class A amp can cost a lot of money. First Watts J2 is a good one. Emotiva has some relatively small heavily biased Class A amps coming out soon. (First 30 watts is Class A then switches to Class B). Most things from Emotiva are reasonable and they have sales every 5 minutes. You just got to keep your eyes opened.
Also what I have my eyes on is the Audio-GD amps. I'm just not sure how much they go into Class A. I can't get a straight answer that I can understand from Kingwa.
Edited by preproman - 9/24/12 at 2:28pm