I'm thinking about using a WC20RC38-s woofer (40$) - although hardly a traditional sub, they can perform well down to 30hz with pretty much no EQ. Combined with a 32w Gainclone amplifier set to double the gain of the PAWO amps, this should provide some pretty decent bass for a very low cost. (I live near Madisound.)

http://www.madisound.com/silverflute.html

Here's a speaker using this exact woofer, or possibly the slightly cheaper non-shielded version.

http://www.tnt-audio.com/casse/jupiter_europa_e.html

I might just build all three gainclone channels into the subwoofer, and use an outboard preamp.