Ack. My bad...

Dammit. I'm having a ground loop and/or EMI problem exactly as described in this thread. I've lived with it a year - it's not noticeable at soft volumes, but now I've decided to get rid of it.

The issue is that I have my empeg playing through a Blitzsafe adapter into the stock HU. I can't see how a ground loop can be happening because all the grounds are short and connected to each other. Ok, so the main ground wire is still the factory stock harness, but I *only* get this noise with my empeg. Tape or radio through the HU is fine. Having the CD selected on my HU without the empeg in the sled is also fine. It is only when the empeg is there that the noise occurs. I've even taken a 12Ga wire and run it from the sled to the battery negative terminal without any effect.

At this point I'm pretty much resigned to pulling my entire dash apart - but I still have no concrete evidence of what the problem really is. The only thought that I have is that maybe the Ratshack M-M RCA adapters between the sled and the Blitzsafe adapter do not carry the shield through. This is the only conjecture that I have that I could see causing the symptoms - that would allow the shields on the HU input side to float and pick up weak EMI.
Assuming that the HU has floating differential inputs (I think this is a reasonable assumption - it's the same HU used in VW's Monsoon option) then without the empeg in the sled both the signal and shield will float and pick up the same EMI noise, hence the noise will be cancelled by the differential input. With the empeg in the sled then the signal is driven but the shield remains floating.

What a pain.