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I'm a big fan of Image Dynamic subs. Get two of these puppies.
i just picked up a 10" pioneer with an enclosure, both off of amazon was like $70 for both. sounds real good anything more would be overkill for me
sticker price on that sub was 250-300 per sub, and when we were selling it out on clearance it was 120. sell the sub no lower than 100 and 150 with box
you said you only got 8" in height? how bout width? i might have a few dual 10 sealed enclosures that might fit. maybe even a 12.
with the measurements you've given and by just eyeing your trunk space i recommend the CP210-W0v3 facing towards your seat
the measurements are: 17.4"(d) x 13.8"(h) x 30"(w)
Sundown SA-8. I don't know how much power you're supplying but this sub is pretty beast.
The box is just as important as the sub. If possible, you should make your own box or get one made. Every sub is different and you want to get the perfect amount of airspace within the box. Pre-fabricated boxes aren't going to be perfectly tuned to your specific sub and the build quality is hit or miss. The difference between a good custom box and a pre-fab is night and day.
I'm lucky because my brother's friend who goes to car audio competitions made me a custom box and plexiglass mounts for my tweeters. But, you might be able to find a place that'll do it for you or maybe you can hit up someone on diymobileaudio.com