Colm4gThe long and short is yes. But not very well.
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The HD650/6XX need 2-4 Vrms, the Nuforce UDAC 5 only outputs 2 Vrms. Meaning at full volume the UDAC 5 would only be able to push the HD6XX to ~108dB. I personally find some movies that have quiet dialog scenes will also have loud action scenes with 120dB peaks that the Nuforce UDAC 5 simply wouldn't do that well for, you either wont hear the quiet scenes, or the loud scenes will be somewhat loud but lacking in bass extension.
So if you listen to things at a lower volume level you might find the Nuforce UDAC 5 is fine. But most people will want more power for dynamic range, especially in movies or classical music.
I'd recommend looking at the following amp/DACs for the HD6XX/650/600
FiiO K5
DAC X6
Monoprice desktop amp
All should be cheaper than the Nuforce.