I cant see how a USB isolator like the one linked to above is any use other than to isolate the ground plane. Hash / noise on the signal lines and power line will still pass through the isolation Tx on each and be received at the DAC.
For a little more money you can get these true isolating boards that will fully isolate the DAC and can be powered by a clean linear power supply. A much better and more elegant solution IMHO.
Listening in a Foo free Zone...
Only a Sith deals in absolutes.