Mission 750LE speaker in need of TLC, advice sought

I have a pair of Mission 750LE Farad Azima anniversary speakers. One of them has developed a problem: the sound has a rather buzzy, distorted quality. It's definitely the speaker, I swapped left and right speakers and the defect followed the speaker. Subjectively it doesn't seem to scale linearly with volume - it's more noticeable at quieter volume levels. The speaker is a 2-way design and the buzzy sound seems to come from the mid / low speaker and not the tweeter.

I've examined the cone and rubber diaphragm and those appear fine, and the coil does not rub against the phase plug.

The other speaker of the pair plays absolutely fine and does not exhibit any buzziness at all. I'd like to get the faulty one working properly again, if that's possible.

Could this be a problem with the crossover network? Or can anyone suggest what else the problem might be and if it's fixable (assume a reasonable level of DIY competence, though I'm more experienced at building stuff from scratch than diagnosing speaker faults).
Take the midwoofer (only driver) out of the circuit and connect it to an amplifier output and play the music while the driver is in a vertical position as if it were in a cabinet and then rotate to see whether the buzzing will disappear. Try the same driver sitting on its magnet on the desk. Crossovers are unlikely to be faulty.