This is the first time we are aware of Windows 7 64-bit BSOD in relation to USB Audio (although googling found this does happens) The HDP and uDAC2 shares the same USB front end, and relies on the Built-In USB Audio from the O/S. Nuforce doesn't provide the driver, USB Audio are part of the OS.
From our previous experience, while not specific W7-64, the USB Audio could get corrupted from computer virus or from system having conflicting or left-over traces of other USB drivers that caused problem.
Try to first do a virus scan to make sure everything is cleaned, then run a disc-scan to fix any potential HD errors, then re-install the latest service pack.