Resurrected after two years... But still a valid question, considering some recent threads.
Nice also to see a mention of the Amity HPA6S. I have an HPA4S (which is the same only with 2 volume controls) and it is astonishingly good.
I have used the following amps with my HD650 (I only mention the amps that did/do well in my opinion):
Musical Fidelity X-can V3 (Pinkie modded + Little Pinkie PSU).
Corda Prehead MKII SE.
Graham Slee Solo.
Rudistor NX33 (balanced)
Rudistor RPX-100 (balanced)
RSA Stealth. (single ended)