If you have the bose system and don't get a new amp you will have to run all the wires from a new head unit and tap them into the speaker outputs of the amp, or rewire all the speakers, which would be a nightmare. I've never seen an amp go for $650, usually around $200-$400 on ebay. I wouldn't bother doing any wiring and just get an amp unless you need to have subs or something.
When I got my 55 someone had a system in it and took it out before trading it in or whatever, they had tapped impedance adapters after the bose amps speaker output but had the input for the amp unplugged for whatever reason. I couldn't find a diagram either. Mine was an easy fix though. Good luck.