Audio System / PC on different A/C circuits

I'm trying to determine the best way to incorporate my PC into my stereo system. The stereo is running thru a balanced step down converter (I live overseas, but use American spec equipment) with a PS Audio P500. My PC is plugged straight to the wall on a separate circuit.

I'm hesitant to use USB since that would tie the grounds. Currently I'm using toslink out from the PC into a benchmark dac1, but the toslink transmitter is on a long ribbon cable inside the computer and doesn't sound too good. I bought a USB to SPDIF converter, but the grounds are tied even though it's using a pulse transformer on the output. My next step is to try a USB to Toslink converter. I have an M-audio sonica, but am wondering if this is the best implementation for my system. I really do not want to plug my PC into my balanced transformer.