Qts is the total q or compliance of the motor. This is a combined formula of qms (mechanic compliance) and qes (electrical compliance).
Low qts subwoofer using have a strong magnet structure. They like ported alignments and larger boxes.
High qts subs like sealed or 4th order bandpass. Smaller.boxes.
To have something overbuilt to handle extreme conditions its going to take alot of speakers and power. Alot of people running large setups like mine are using pro audio woofers. Generally a 8-10" mid and a buillet tweeter. You can run less speakers this way and still be very loud. For the average person sound is ok, but for audiophiles sound quality is sub par.
If your wanting decent bass you gonna need to ditch the seat.
4 8's sealed or 2 8's Porter could fit under the rear seat but if you can deal with my truck you wont be satisified.
My questions for you...
What type of music will the system mostly be playing?
Any certain level of loud your goal is?