rlcowboy gave you about the best advice so far. Most of the guys told you something like "you have crap equipment so that's what it's going to sound like". Well, obviously you allready know your equipment isn't that good. I would take rlcowboy's advice and upgrade your subs and box first. Buy the subs first and try putting some dacron or polyfill (pillow stuffing) inside your enclosure. One of the guys mentioned half-filling the enclosure. That's the right thing to do. Also, already mentioned, your gain and crossover settings are very important. IM me and I'll talk you through adjusting your gain settings. There isn't one magical brand of subwoofer to go with. JL Audio is a good sub, Treo makes an awsome sub, even Powerbass is good if you want to save some money (not as good as JL or Treo but still not bad for the money). The Treo TSi is their entry model sub and it retails for about $150 or less. I could recommend a dozen or so other subs that would work fine. These are just the first couple that came to mind. Treo makes an awsome amp too. Feel free to send me a message if I can help you out. I hope this helps.
Steve