Can you put 2 of the Jl 13's under the seat? Sorry to jack the thread. I have the factory sub as well but i want exceptional hearing loss.... my grandpas already deaf im not too worried.
I don't think you could put a pair of them under the seat as the single goes to the drivers side where the floor bumps down.
For those with power sliding rear window, and minimum interior mods, then I think you are stuck with the single JL as the best bet.
For those without power sliding rear windows, for the biggest boom without the least bit of interior modding, I would run 4 12" Pioneer shallows in a sealed box behind the rear seat. Feed them about 1500w and it should get pretty loud.
Now for the fun, people who are willing the mod the interiors, the possibilities are endless MUHAHAHA LOL
For flatout loudest possible while mainting all seating, you are looking at doing a blow through setup, done right it could be hidden with a diamondplate box, reach 150+ db's on the meter and look stock to most people looking in.
Now for those who don't want to cut up the sheet metal you are going to have to move some seats, either use up the front center console area or change to rear captains chairs and build a rear center console.
OR, remove the rear seat altogether and wall off the back, with this I think you can squeeze in 6 15's or 18's with proper airspace and lots of port area, but sorta impractical LOL