>>the particular noise I have is attuned to the engine; it gets loud when I rev, goes to a minimum when the car's idling.

>By that do you mean the frequency of the noise increases, or the volume of the noise increases?

Now that the empeg is back from its journey home, and I've been away for a wedding 2 funerals and another trip, I can come back to this.

It's frequency.

At this point my list of things to try is power direct from battery (probably not today since it's raining) and ground loop isolators. Bearing in mind that I already tried a power filter, and feeding the externally-powered empeg through the same RCAs produces no odd noise, if that fails I'll be at a complete loss.