The way you describe your component chain, you are running a Nuforce uDAC-2 into an unspecified Yamaha A/V receiver, which is what the HD650s are actually plugged into? Try this as an experiment: use the 1/4-inch-to-1/8-inch three-way stereo adapter plug that comes in the box with each pair of HD650s and plug them directly into the uDAC-2's headphone jack, rather than using the RCA or S/PDIF optical cable connections on the back of your Nuforce to connect it your Yamaha. Look at my equipment inventory in my profile, and read this old post of mine answering another Head-Fi-er's question:
I think it will sound better than plugging the HD650s into whatever kind of Yamaha unit you have. Typically, A/V receiver manufacturers "hang" a rather large-value resistor on the headphone-output jack, because most people use their speakers most of the time, which consume much more current from the amplifier output stage. You will probably be like me, and keep your uDAC-2 volume pot at the max-ed out position all the time, but in doing so, I never heard any clipping. The HD650s are a nominal 300-ohm impedence phone, and being powered off of a 5-volt USB port, the uDAC-2 simply cannot deliver enough voltage into that kind of load to clip. You will sometimes wish you could play some types of music louder (particularly things with wide dynamic range or a lot of deep bass content), but I will almost bet money it will sound better to you quality-wise than plugging the HD650s into the Yamaha and using it as your effective headamp. The Nuforce uDAC-2, like their higher-end products, was synergystically designed to be both a DAC and a headphone amp; the Yamaha was not.
Try it, and let me know if I was right or wrong (I am not the least bit afraid of being the latter, including in public).
Actually, when I first opened the HD650, I'd done just that, but it just didn't go loud enough, so I opted for the Yamaha add-in, which I connected from the uDac2's headphone jack to Yamaha RCA input (probably a dumb noobie move). See, I also made the same mistake of getting the $99 uDac2 because I didn't think of the scope of the hobby, nor did I think I would be too limited by the volume pot. But that's the beauty of hobby: we can upgrade, at the expense of, well, expenses. I learned my lesson, and I'm glad I'm doing so early on.
But you're very correct: it sounds cleaner the simpler route , and there is no clipping. And honestly, the maxed-out volume isn't so bad. I can make do until my next upgrade.