The factory sub is garbage....Pull that out and put an aftermarket sub in the stock enclosure. I put in a Rockford Fosgate 8" that was like $80 and it sounds really good. I mounted an old Phoenix Gold amp under the drivers seat and took out the factory amp that was in the sub enclosure. The factory sub feels like a paper weight in comparison to a real sub and if you put any kind of power to it from an aftermarket amp, that thing is going to blow quick.
all you need is 1 12" sub well powered. Trust me. I have had multiple setups and installed multiple in other vehicles. In an open SUV, one is enough. I miss my 12" Infinity Perfect and 350watt Alpine amp I had in my previous SUV.