Ok Brian, here's an easy way to determine what is wrong.
To check the surrounds, simply take the grilles off of the speakers and look closely at the foam surrounding the speaker, inspecting for cracks/breaks, etc. Also run your finger around the foam, as sometimes the cracks are hard to see until the foam is moved.
If you find the foam(s) to be cracked and/or broken, you have probably found your problem. This will typically cause a "buzzing" sound during bass notes, making the speaker sound blown.
If the foams seem to be intact, the next step is to give a closer inspection to the woofer. Place your fingers around the cone of the speaker (roughly spread around the center in an even dispersion), and gently push the cone inward with your hand. Take care to push it straight in. If there is a scraping noise or if the speaker doesn't want to move, it probably is blown.
So, if you find the speaker to be blown the next step is replacement. Because of your location/budget constraints I believe this is the best remedy possible. The best course of action here is to find a direct replacement (i.e., the same speaker) to replace it with. Once you have gotten the replacement woofer, it is as easy as unscrewing the broken one, unattaching the wires, attaching them to the new speaker, and screwing it in. Infinity doesn't use solder, so you should be able to simply pull the connections from the speaker and directly attach them to the new one. Then screw the new speaker in, and you're done!
I think the best action for you to take if the speaker is blown is to e-mail Infinity's (Harman actually, their parent company) parts administrator at
parts@harman.com. They should be able to help you find a replacement woofer.
Sorry the post is so long, but I hope it helps!
Good luck Brian!
Jeremy