Is there an amp in this stero system behind the head unit? If so what watts is it per speaker? Will a HU make it louder than this amp? Will be adding subs on day.....

 

If you have just the standard stereo setup (non-mach), you have the stock HU, and 4 speakers. Thats it, no amps or factory subs.

If you get an aftermarket HU, it will put out more power than the stock headunit, but what youll really want to do is add an amp for your speakers (which you'll have to do if you plan to add subs anyway).