I have 2 Infinity Kappa Perfect VQ subs each with .6 cubic ft behind
them, and they sound beautiful. (In a civic, and I still have pleny of
trunk space.) Just make sure you put enough power to it. For a small
system one would be fine, honestly, for my needs 2 was overkill. The
acoustic accuracy of these subs is truly inredible, and they are
EXTREMELY versitle (genius pole peice inserts). They are a bit pricey,
but you can price match @ www.sounddomain.com thats where I got mine.
You can probably get one for about $250. They can be put in sealed
enclusures as small as .4 cubic ft.