Sounds strange that multiple tweeters would all be bad.
Have you tried swapping the tweeters between the two M80s. If not take the two known good tweeters out of the other M80s and put them in the bad one. Also, put the tweeters from the bad M80 (especially the new tweeter you just got) and put them in the good M80. If the problem follows the tweeter then it is actually bad. If the problem stays with the speaker itself then it’s not the tweeters but something else in the cabinet that’s wrong.
Have you check where the wires come off the crossover to make sure their not loose there and not just at the driver itself?
