the setup i have sitting in my garage is a nice way to keep the head unit and get as much power as you want.....i used line level converter on the speaker wires from the head unit (not high to low converters) and ran those into a preamp EQ. i have 2 sets of RCA's running in (front and rear) and 3 jacks running out (front rear and sub) that i ran to 2 amps. 1 for subs and the other for the highs. i started piecing this system together to keep from losing steering wheel controls and so people dont see ANYTHING when looking to break in. i like a stealth look.

 

Bingo- that's what I'm lookin' to do. So nowbdy knows nothing when they look thru the window......

 

One trick we have done on the bose systems at work that works quite nicely ans sounds awesome is to put a line out converter on the front and rear outputs out of the bose amp and run to a high power aftermarket four channel amp for the highs and then wire from the amp back into the stock harness and power the factory speakers. It will be clear as hell and loud. Then for the sub disconnect the factory wires heading to the sub and put another line out on those wires and run it to an aftermarket amp of your choosing and a well built box and sub.