I have a 2002 WRX with premium factory sound and the factory powred sub-woofer under the passanger's seat.

I'm thinking of replacing the head unit with one that reads SD cards, because playing MP3's through the cassette adapter = teh suck. (the only one I've seen so far is a Blaupunkt.) What I'm not sure of is how the speakers are wired? are there 4 speaker channels? does the sub have its own output? I'm just trying to figure out what kind of outputs the new head unit will need to have. oh, and how much power does the OEM head unit put out