I've been thinking about the amp/headunit switching thing and it seems like there's two ways (not counting the hardwire CD input mod which breaks the CD player technique). The first, lower complexity, lower quality way is to use and FM modulator on the empeg's output and feed that into the FM input on the stock system. This leaves most stuff alone. The other way that I can see to run two units is that you'd need the Sony 2-head switching unit (or similar), plus speaker to line level converters for the output of the Subaru HU. The out from the Sony switch would then feed your amp(s). You'd also need a voltage sensor as the Subaru HU has no amp turn-on line as far as I can tell. If I did it, I think I'd try the FM Modulator system first. If it didn't work I'd only be out a few tens of dollars.

-Zeke