Give that man a coconut! I think it must have something to do with this:
Does the AQVox separate the ground from the computer and AP2? Like you said, the PP part powers only the output section and clocks, but the receiver still relies on bus power. The AP2 unit supposedly provides galvanic isolation, but how well such a small unit could do that I'm unsure of. Does driving the SOtM with battery isolate the the power rails only, or does it lift the ground internally from the rest of the computer? (I don't think it should be able to do this.) If the the AQVox is doing something to provide another solid ground between the two devices or completely disconnecting the grounds then it may be able to remove some noise from the system.
Reason: I was surprised this morning to hear - whilst no music was playing - a little bit of mains/mobo noise coming out of my speakers - inaudible from the main listening position, but there nonetheless. I had thought that I'd resolved these issues a long time ago, so I was a little perplexed. Then I remembered that, once again, I had unplugged the AQVOX. I plugged it back in, and voila, the speakers were silent. I suspect, then, that there may be something to the idea that the AQVOX is, as you say, either providing a solid ground between the two devices, or completely disconnecting them. Either way, I guess it may end staying.