I'm going to throw the Burson HA-160DS, despite the fact that it's above your budget. If you save enough, it'll give you a DAC and an amp that pairs very well with the HD650. I'm using it in my rig and the more I listen to it, the more I'm forgetting about the gear and just listening and grooving to the tunes. It's really giving me a final nail in the coffin on requiring anything additional (READ: Upgraditis!).
The HA-160 is just the headphone amplifier product from Burson but it should have similar characteristics, although not sure what DAC you'll be using it with.