Earlier quoted context omitted.
I'm not a pro audio guy, but I think if you want to work with any external audio device and want that connection to be noise-free, your safest bet is still optical. I have an external DAC/amp, and I had been using optical until I recently upgraded my workstation. Now I am using USB, because the new motherboard doesn't have optical; I assumed USB would be fine, so the lack of optical wasn't something I considered with…
Since it’s USB, it has to be the DAC generating the hiss. No amount of electrical interference can do anything to the digital audio going Ofer USB.
So it's the DACs fault, but maybe it's fair to say that it's harder to make a good DAC when USB is how the data is being delivered. I'm sure a high end USB DAC doesn't have this issue, but I'm pretty impressed with my cheap offbrand optical DAC that was $11. I'm guessing I would not be so impressed with the $11 USB version.