Well... in that price range the Millett MiniMax is superb with Senns (and there are some for sale on the FS Forum, as well as a Millett Max). Though, with the "new version" HD580/600/650's, these amps could be a bit aggressive in the upper mids and highs. Given you're preferences, they might best be paired with the "old version" of these phones.
Another group of amps in that price range that are great with Senns: the X-CAN v2 and v3... and the DV332.
Of these I'd likely give the strongest recommendation to the DV332 (with the right tubes)... IF... you're not going to be driving low impedance phones (e.g. Grados, ATH's, Denons, etc.). Both the Milletts and the X-CAN's drive both high and low impedance phones well.
I second Gradofan's suggestions. I've been Using an Icon HDP very successfully with recabled 650's for about 8 months now. However, after demoing many amps (TOOBS!) (a 16k McIntosh setup in particular) I've found it's time to unleash the HD 650's for and in my own home. There are a few Great DIY kits out there that will compliment HD 650's superbly within 200-300$.
I'm about to build a Millet Hybrid Minimax, and I am stoked for it.
Not only will the build be FUN! But over the years I've not seen a negative or less than splended review of this amp. Although I don't have one myself, (until next week) I'm quite confident in it's abilities. Especially for the money!
People tend not to put the minimax down for dynamic headphones until they reach Burson HA-160 levels and the like. One review would be a titch skeptical, but they're all that way... Another guy didn't sell his Minimax until he got a 2k Melos. Another only ditched it for a Graham Slee Solo. Another one of the high end Woo amps. (I don't remember the model)
All I know is that there seems to be unanimous love for the minimax. It's a complete kit, with high quality componets, and the new kits from Beezar are nearly un-upgradable. The customer support is also excellent. While learning about it over the last couple days I've exchanged around 16-20ish pm's.
But the best part to me is that since the only DIY I've done is 3 cables, I love how complete the Kit is. It's a great place to start and be satisfied for quite some time. It's small, compact, and a value killer. Solid state/Tube buffered amp. A great combo in the price range and the pinnacle of the Pete Millet evolution. Needless to say, I'm excited. Let the tube rolling begin!
Enjoy the hunt, anyhow! HD 650's on the right system can blow most anything out of the water, or at least compete.