Here's my 2 cents. I recently revamped my system in my 89 Chevy 2500 ext. cab. I replaced the bench seat with buckets and made a middle console downfiring box with future amp rack goin in on top. I know you have a box for 12's under the rear seat, but i was going for wow factor and got 2 Sundown SA-8's and went ported, and "wow" was the result. You say you dont care for loudness or "wow" for that matter, so im recommending what smgreen20 said, make an 8" baffle and stick a pair of 8's in there. DD's 8 inch subs are nice, but are optimal in ported apps, Sundown's 8's are nice but are also optimal in ported apps. Soundsplinter RL-i8's are supposed to be really amazing 8's that can sound GOOD in sealed and probably even better ported. You have more than enough space to house two Soundsplinters, or maybe even any of the subs i listed, 8's require very small amounts of airspace compared to 12's.