NuForce HDP owners seem to like it with the HD650. Don't endorse the idea but with the HDP you can buy it from Amazon, try it and if it's not to your liking you can return it. That's another thing the HDP has going for it.
Also, the Matrix M-Stage is liked by many and is designed (almost a clone) around the Lehmann Black Cube (which sells for upwards of $800) and costs about $260, little less used when it comes up. There's the option of opamp rolling with it too.
Lengthy thread on it: http://www.head-fi.org/forum/thread/475618/matrix-m-stage-amp-review-simple-cheap-and-excellent
There's also the Lovely Cube (almost a direct Lehman clone) as well and sells for $245 I think.
Then there's Woo Audio (tube based) and DIY stuff like the M^3.
The Burson HA-160 costs more but is very well liked with the HD650 from what I've read. These are just to name a few. You can do a search and find much more, the HD650 gets asked about a lot.