I think I might have found a winner:


It's a switcher, not a mixer, but the delay is configurable from half a second up to several minutes, and the audio sense level is configurable as well. It's 12V DC @75 mA, so I should be able to just drop it on the amp remote line, which I already split out to power my XM direct receiver. And it's only an inch high, so it should fit easily under my carpet where the XM receiver already lives.

Looks like they can be had for about 60 bucks.