I have an Emotiva BDP-1 (i think that is the model) that does a good job with them, and my vintage Marantz 2240 is even better (slightly warmer and punchier but not as detailed and spacious). Both were under $200. I think the EF5 amp is optimal with the HE-500, but it pushes your budget a little bit.
I don't see the BDP-1. Must have it mixed up with something else?
What I'm really looking for is just the O2, but with more power. So something with an extremely low output impedance, low distortion, low noise, and flat frequency response. I've always found vintage receivers to skew more towards the warm side of things, which is not what I'm looking for.
If I could get ahold of the PSU and SE-SE boards for "The Wire" amp, I'd build one and be ready to go.
Edited by evanft - 7/10/12 at 4:21pm