Power supply is only really an issue for those few extreme audiophiles. The design of a unit has to be really seriously incompetent for the power supply to affect the digital audio data itself. It's entirely possible that a power supply could introduce some jitter but nothing which should be heard and certainly nothing that could be heard if the DAC's jitter reduction circuitry is functioning correctly. Top class jitter reduction circuitry probably costs around $30 and good enough, probably half that.
Those USB-SPDIF are obviously useful for format conversion but when it comes to jitter they are effectively curing a problem which doesn't exist. As jitter is removed by the DAC.
Thanks - wasn't sure about how that worked. I know more about how sensitive some embedded systems (cars, mobile phone networks, etc) can be when it comes to power supply, so that's where I was coming from.
I'll make sure to borrow some converters rather than buy them, as I'd have a hard time believing the Nuforce HDP DAC is flawed. (Classic 'my new stuff can't be broken' defense!)