You can "build" a very nice sub for around $400, using prefab boxes, plate amps and DIY drivers. I put build in quotes because these are essentially kits that one bolts together-there is no real work other than mounting the driver and amp to the box, wiring up the two and connecting it to your system. I "built" a very nice 12" sub with an NHT 1259 driver, a Rhythmik (sp?) Audio A370 amp and a Madisound 3.3 cf box for around $500, which IMHO is a giant killer of a sub-F3dB is around 25 Hz, which is pretty good at any price. Mine is a bit over your price, but there are similar kits for your price point (and less) that will give you very high quality performance if you are into the DIY thing.