Met, welcome. Bi-wiring(sometimes sarcastically termed "buy-wiring")isn't significantly different electrically and can't provide better sound quality. This has been discussed at length before, but briefly, the only difference is that instead of the wiring dividing just before it goes into the separate high and low sections of the speaker crossover inside the speaker(the wiring has to divide at latest by then)it instead divides as it leaves the amplifier terminals, but this has no sonic significance.
Now, although there's no benefit, there should also be no harm involved in the process. It appears that the mid/treble section of one M80 somehow became disconnected. If you're sure that the connecting strap was tight when you went back to the single wiring(re-check it)then the problem is internal, likely just a loose wire(but why?; just a coincidence?).If you want to open it up, you could examine the wiring. But no, if you're concerned that this somehow burned out the drivers or the mid/treble section of the crossover, there's no reason for that to have happened.
