I have the same problem in my single cab, but i have the bench seat blues. You can fit 8" subs behind the seat and still go all the way back. I have two Memphis M Class 8's and the amp is under the seat. You cant fit a very big amp under the seat, but there is room, and some time you may have to get creative. If you are starting out with nothing kicker makes a few subs called the CVT's that are low profile, and come in some really nice enclosures. You can get them in 10's and 12's. Pioneer also makes a thin woofer, and so does clarion, and JL audio. Memphis has just released a new thin woofer, but they are hard to find at the moment. I would lean towards the kicker CVT in the pre-fab box with a nice mono amp of about 400 watts RMS.
