Is the USB to S/PDIF optical or co-ax?
If it is co-ax, then the Isolator will block noise by breaking the ground loop.
If it is optical, then there is no ground loop anyway.
The ADuM3160 does clean up the signal to some extent...it doesn't just
copy the signal ad-hoc from one side to the other.
Of course, whatever chip runs the USB to S/PDIF conversion (PCM270x maybe?)
could possibly benefit from the better power supply.
In the end though, I would make no guarantee of any audible improvement, as
this is not really the kind of device it was design for.
I use a musical fidelity v-link (which uses a TAS1020B chip?) and connect it to the DAC though optical out. Think it will be interesting to see how much of a difference the Doodlebug can make.