like people have been saying the whole time...you still havent said what your aiming for.
sq, spl?
personally with 500 bucks i would buy a xxx for both sq and spl (the 04-05 models)
for spl you could do the btl
or for more sq but still getting pretty **** loud you could go for a magnum
only sub i dont have experience with is the btl, so take that suggestion with a grain of salt.
did i mention my friend is selling his 05 15" xxx.....