"so far there is no high-pitched noise when using the stock PC on the ARC. Perhaps the Audience PC is not a good design for a digital device?"
Now that sounds decidedly odd just out of curiosity are you using a cheater plug with both the Audience PC and the stock power cord? Also just wondering what's the output voltage of the ARC? It could still be a gain issue, not saying that the IEC isn't wired incorrectly goodness knows jea48 knows a lot about that subject, but an interactive issue would seem to make more sense equipment mis-design? Or at least more likely considering you've had the ARC long term with no problems until the new preamp showed up.
Now that sounds decidedly odd just out of curiosity are you using a cheater plug with both the Audience PC and the stock power cord? Also just wondering what's the output voltage of the ARC? It could still be a gain issue, not saying that the IEC isn't wired incorrectly goodness knows jea48 knows a lot about that subject, but an interactive issue would seem to make more sense equipment mis-design? Or at least more likely considering you've had the ARC long term with no problems until the new preamp showed up.