OK< I am considering a new upgrade for my 2007 GL1800. My installer will build custom fiberglass box into side saddles, with 6" sub woofer. This will take up approximately 20% of the saddle space. Perfect for me. Then install Rockford Fosgate marine amps to drive the subs, and 4 new arc audio speakers for front and rear. My question here is, can the factory amp speaker outputs to speakers be converted to line outs with converter as input for new amp? Or, are the outputs from the head unit line out signals already?