You wanna go behind the seat build, or console!?
If you wanna go behind the seat, w/ out doing a blow threw- id go w/ shallow mount subwoofer's.
Kenwood just came out with a really nice one, pioneer, kicker, clarion all make a nice shallow subwoofer along with rockford, im running one of the pioneer's in a 1.0^ft enclosure with a 3/4" port, and it sounds really good, i have about 500rms going to it, how ever its in a smaller truck than yours...
If you wanna go w/ the deeper subwoofers, i have seen some single cabs like yours with diamond audio D6 subs, ive heard alot of good stuff with them, and they sound really good with the amount of space you have to work with. You said you dont wanna go all loud and SPL type stuff, you just wanna get decently loud, so 1 or 2 12's i think would serve you well. Maybe even 2 10's w/ the right set up.