So far I've been really impressed with the Yulong A800 mini Class A headphone amp paired with my HD650. For $180, this amp has pretty much everything I need from a budget amp: clarity, neutrality and power. The dynamics are excellent. Good soundstage, too. So far, I've owned the E9, P4, Asgard, O2 and the AHA-120. I previously held the AHA-120 as the best of the lot. I think the A800 is at least as good and beats the other amps. Its a really clean amp. Yeah, its still a budget amp but I think it performs better than its price tag with the HD650.
Thanks for the impressions. I was curious about Yulong's new class A mini amp myself. Do you find it works better with high impedance headphones than low impedance headphones? Or does it not matter?