Ok, that's easy enough. Lets go with $300. That should give you a decent amount of bass.
Try looking at this bass package:
Rockford Fosgate Bass Package Punch P300-1 mono subwoofer amplifier Two Punch Stage 1 12" 4-ohm subwoofers at Crutchfield.com This costs a lil over $300
Or if you want a custom set up.
Try
2: MTX Thunder XT10-04
and a profile ap600 amp.
or for one sub
A Kicker Comp VR 07CVR124
and a Kenwood KAC-6202 amp.
This gives you a variety of brands to play with.
for all of these, i would get a crossover too, even just a cheap set up.
Hope this helps.
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1998 GT
Full Flowmaster Exhaust.
18" bullitt style wheels
JVC Deck, Rockford Speakers, and a polk sub.
More on the way