In this small a sound stage, there is really no need for a massive 10" or larger woofer... a modest 8" low profile unit will suffice neatly beneath either seat or in the rear deck lid.
I am going to add modest rear speakers in the rear deck lid to add depth to the sound stage and probably upgrade each of the factory speakers to better quality components and perhaps a mild amplifier to drive the coils.
Building a SPL test system, is way out of my desire... I need, prefer, clear "presence", realistic depth, and mild amplification to the subtle sounds like the cartridges hitting the floor in "The Matrix"
Loud is not always better... but the ability to hear the 105mm cannons in the 1812 overture, in real world impact, is nice to have...grin
BUT..... Beware! While I can appreciate quality amplified music... sharing your deep bass, and only your deep bass thump, with me, from three blocks away...... is bound to make me irrate... and I am legion!