Being a bass guitar man that i am, i could suggust geting either a 15/2x15 cabinet, and having it run by your amp (look used, its cheeper). or, if you want a powered sub, go for taking the woofer line out and pluging it into a guitar/bass combo amp or head (preferably a head, or if your tricky, a rack mounted system could be fun to toy with). I think that this would be cost effective because i got my bass rig used, and it cost under 500$... 2 15" cones, and 400 watts with 9 band eq shelving for high/low, etc... and if your tempted, almost all amp heads have a line patch or a crossover, so you coud send more to another woofer (at different frequency crosses). You can do fun stuff with it. to add to it, you can get a guitar or bass and plug it in if your ever intrested (guitar works on bass amps, bass doesnt work on guitar amps)
how often do you see a woofer with an eq?