My guess would be that with the cone area from the 2 12's you would still be better off from a SPL point of view if you power them right. If your wanting more space you can always build a nice ported job for the one Kicker, of course it may end up being just as big. You would probably gain about 3 decibles per sub though if you ported the 2 12's. Like you said, with limited space, its not always that easy. I personally feel it would be alot less hassle just to leave your 2 12's in a sealed box, and power them well with a nice amp, like an orion 1200D. Kicker also makes some devastatingly loud SX series amps if you wanna stay with the brand. You wouldnt have to worry about bulding a ported box just right for the other, and it would be tight as hell if you can get rid of all the rattles. Of course if you build and tune a ported box just right, the loss in "tightness" should be negligable. I think it really depends on how much time and effort you want to put into the setup. Unless your a stickler for SPL, I would stick with the sealed setup, and invest your money in a good comp set, sound deadening, and CD player with lots of tuning options.
