Ive got a typical 4-door Protégé. I currently have the sub in my trunk in a box that is honestly crap but works for now. The amp I have was sold to me by a co-worker for 60 bucks. At the end of the build I had the two 12"s, 600w amp and my enclosure for about 150$. I don't think that's bad but obviously I get what I pay for. So that being said, I will take a look at getting a more powerful amp. After that ill look into getting some better quality subs, then the box to fit it. Seems like the proper approach to this all.
I'd love to get a pair of 8"s, 12"s, then 15"s... but that's something I don't think I can do myself... let alone possible in a trunk... sigh... car shopping anyone?