I think the digital signal part has already been addressed well. Regarding the power part, I can say this. One, the USB power is considered to be one of the cleaner sources out there.
Secondly, digital chips work within a certain min/max range. If you look at the DAC datasheet, most of them use VDD as reference. So it doesn't matter what VDD is, as long as its between 3.3-5V. Thats a big range, even if you consider HF noise. So unless you USB cable is so bad it cannot even transfer 3.3V reliably, it should not cause an issue.
Whoa wait a sec. USB power from most computers is clean? Compared to what?
Secondly, even if a lot of chips can operate fine with a range of supply voltages, some may be preferred over others. A DAC is an analog chip anyhow, at least for the purposes of conditions needing to be more exacting for good performance. And certainly if there's enough noise on the supply rails, that impacts performance. What do you think anybody lists or advertises good PSRR?