Kriskros is right.

Building a subwoofers system there is a PROS & CONS approach.
PROS: Two 12" subs more bass output.

CONS: limited space, big heavy box, more gas in your car for your heavy loads. Not to mention if your charging system could handle additional with the needed current and voltage to operate properly.

Try to envision what kind of system is enough for you to be required to put the pieces in your system. DONT BITE MORE THAN YOU COULD CHEW. Money wise is a must unless, total amount is not an option.

DYNAMAT is the best deadening for your car to reinforces your trunk to help reduces noise and rattles and dont forget also to use HushMat sealing caulk and tape to dampen noise in corners and hard to-reach areas. They dont come cheap so you better prepared to put a hole in your pocket.