I'm preparing to order an 07 GT soon, and was wondering how reusable the Shaker 500 system is. I'm planning on putting in my own head unit to replace the stock stereo, and my head unit does not have an internal amplifier. Does the Shaker 500 use external amps for all the speakers, or just the subwoofers? And in either case, how hard it is to run a 4v line level signal to them, as that is what my head unit provides.

For those interested, the head unit I have is an empeg-car MP3 player. It's been in 3 cars now that I have owned since it came out, and I have gotten so used to it that it has to go into my future Mustang as well. More info on it is here.