honestly,
- take that PAC junk and crappy stock HU out of the car.
- throw in a decent nav with an equalizer (like you are already planning to).
- put some matting in the interior (i use raammat)
- throw the 8's in the trash, you may be experiencing some cancellation
- set your speakers on high pass crossover through the nav
thatll clean up the sound a LOT and the bass will feel a lot better.
treo is an excellent product. i recommend giving that shot because it wont cost any extra over your current plans ,with the exception of the matting. i think your tuning, the shitty output from the shitty stock headunit, and vibrations from the car may all be problematic currently.
theres no reason to go full retard with a megabox in the back. one strong 12, tuned well, with the rest of the car worked over to match, will sound excellent.
i used to run a resonant engineering SE12 in my old truck, trying to find a good box to use for the current car. that thing got retarded loud when i was feeding it 800 watts RMS. i'm absolutely sure it will be overkill for an s197gt.
sure you can keep cramming woofers in there, but theres only a certain amount of bass you can enjoy before it ruins the rest of the setup.
i wouldnt even mess with polyfill, its not going to bandaid a setup enough to make a noticeable difference.
try the treo first. if redoing the rest of the car and properly tuning the treo isn't enough, save space and buy one higher quality 12 with some ridiculous excursion and depth. a quality 12 in a sealed box can still deliver tight bass while going much lower than a 10 can ever dream of.
at this point, since he wants bass, i wouldnt recommend bothering with spending too much on speakers. get a 100-200 dollar component set for the front (alpine type R's are great for hte money) and run an amp.
