FWIW, I've got a pair of HD 600s and power them with a NuForce uDAC. Many said it couldn't (or rather, shouldn't) be done, but I'm happy with it. I spent some time comparing it to a Beyerdynamic A1 amp, and after matching levels by ear I couldn't consistently tell the difference between them at any volume. The Beyer amp may have been slightly "warmer" sounding - but the difference was so small that I could never ever justify spending the $850 one would cost. It was not a blind test - just me A/B'ing them - so take it as you will.
Of course, the HD 650 is different animal - and I was using a USB-powered amp. But this should give you some perspective.