One you can do is remove the Sub from the speaker box. Attach speaker wire back to the sub. Take the positive side and hold it to a cordless drill battery (Preferably 12volt or 9.6, if neither is available, just use a 9volt battery people use in firealarms). Take the negative side and hold it to the negative side of the drill battery, for 2 seconds. If the sub 'pops' or moves outward, (if you have the polarity backwards it moves inward), then your speaker still has life in it.
This doesn't mean the sub is 'not' blown, but it is likely you are having problems outside the sub.
I'm late to work, so try this, if it doesn't work, post it back in reply. We'll figure out whats wrong with it.