2007 gmc acadia

I have never encountered a stock system (bose or otherwise) that had amp preouts.  If you're stock system is factory amped, the amplifer will be recieving a low-level signal from the head unit but that low lever signal is being transferred via speaker wire, not RCA's.  In order to hook up the subs you'll need to purchase a line out converter to give you a sort of makeshift set of RCA preouts (or, if your MTX amps have high-level speaker inputs you could use those)  You may want to consider replacing the stock head unit with an aftermarket one to get better bass and better bass control.  I know you probably don't want to give up the factory Bose head unit, but I've replaced those before, and replacing it is something you needn't worry about (as far as sound quality is concerned)so long as you replace it with a good SQ deck such as Eclipse or Alpine.