Nick, sorry to hear of the difficulties you're having. Although you emphasized that the wiring isn't the problem, since you didn't specifically say so, I'm going to ask if you took the speaker wire from the other speaker and connected it to the silent woofers to make sure that the wire to them didn't develop a defect.
If that's not it, it can be assumed that two woofers didn't simultaneously fail. That would leave the internal wiring from the input terminal to the crossover or the low frequency section of the crossover itself. If you can open up the enclosure to see if the internal wire has vibrated loose, that would check that possibility. If it's the crossover itself, that would require a call to Axiom for replacement and listening in mono in the meantime.
