Logically, it seems like if you had a USB-optical dongle you would have no further trouble. I don't know if there is such a thing, but I do know that in that situation all the 'it's digital, so it should be perfect' talk becomes somewhat true.
You'd be converting from USB to optical, at which point you'd break the ground connection which would be where your hiss is coming from (assuming it's still noiseless when still being used with optical). Then, your concern would be jitter and whether the added conversion is adding lots of jitter to the equation. Your DAC might (or might not) be good at rejecting jitter noise. I've got a Lavry DA10 that's exceptionally good at rejecting jitter (in crystal mode), but that's mastering grade and maybe overkill for you.
It wouldn't add literal noise, but it's also possible for the USB connection to be more jittery than a different computer making the optical connection. That's partly hardware and partly software design (controlling how the data stream is buffered, and associated things that might slightly modulate the audio data clocking). So a change in computer feeding the DAC could also substantially affect the 'sound' of the DAC, as well as the noise issue you observed.
I agree, that should work. I haven't seen USB-optical devices that weren't straight media converters though. Everything that converts USB to spdif is advertised as a DAC, so I'd have two DACs.
This isn't a huge issue for me, it's just an example of where I'd prefer to use optical. If it ends up bothering me such that I need to fix something, I'll 'fix' the motherboard.
You'd be converting from USB to optical, at which point you'd break the ground connection which would be where your hiss is coming from (assuming it's still noiseless when still being used with optical). Then, your concern would be jitter and whether the added conversion is adding lots of jitter to the equation. Your DAC might (or might not) be good at rejecting jitter noise. I've got a Lavry DA10 that's exceptionally good at rejecting jitter (in crystal mode), but that's mastering grade and maybe overkill for you.
It wouldn't add literal noise, but it's also possible for the USB connection to be more jittery than a different computer making the optical connection. That's partly hardware and partly software design (controlling how the data stream is buffered, and associated things that might slightly modulate the audio data clocking). So a change in computer feeding the DAC could also substantially affect the 'sound' of the DAC, as well as the noise issue you observed.