I keep recommending these subs to people because they take super small sealed boxes, play really low, and have excellent sound quality. They are the Image Dynamics IDQ v.2. We did a single 10" in my brother's Maxima. The box volume was 0.65 cu. ft.. He powers it with a mere 150W Sony amp and it sounds pretty good. If he were to go to 300W, it would perform even better. The 12" requires only 0.85 cu. ft. to function properly. I would recommend going with the larger driver to get deeper sounding bass from it.

The problem you are going to run into with your 4x6 + bandpass sub system is the HUGE hole in the mid bass frequencies.

To get the sound to the cabin of the car, totally remove the bins in the rear deck, cut the lids off the deck and create a custom grille cover to hide the hideous holes.