Smaller subs..

i'd recommend an 8" kicker L5,a little pricy though but they sound great

resonant engineering re 8's just google em they are loud, clean, and cheap.

I will be running those new shallow mount Pioneer subs in mine when im ready....I heard 1 8" sub in a Ferrari and it sounded really nice....good tight, clean bass....

the 8" L5s are pretty damn good for an 8" sub we have one in my boys hardbody

Elemental Designs EU700s you could put 3-4 in there and maybe even in a ported enclosure. It would sound quite nice.

I agree with the RE's, a shop here in my town carries them and they are the best product made in my opinion

I heard RE does best for small subs

i got an 8inch jl audio sub in my dads z3 between the roll bars it sounds pretty damn nice even with the wind noise

jl has new 6.5" sub coming out soon and can use very small ported box also.
check their new page:

http://mobile.jlaudio.com/jlaudio_pages.php?page_id=214

6W3v3
Nominal Diameter: 6.5 in. (165 mm)
Continuous Power Handling: 125 W
One-Way Linear Excursion (Xmax): 0.35 in. (8.9 mm)
Nominal Impedance: Single 4 ohm or Single 8 ohm
Mounting Depth:?4.25 in. (108 mm)
Sealed Enclosure Recommendation: 0.18 cu.ft. (5.1 liters)
Ported Enclosure Range:?0.20-0.30 cu.ft. (5.7-8.5 liters)

This is a dd 1000-8 will take about a 1000 watts and hit like two jl w3 12's I know sounds crazy but I have seen it. DDAUDIO.com