Yeah, Rick; I remember the tweeter disaster. Maybe you shouldn't play at 150dB? But more seriously, if you listened from up close and it's definitely coming from both woofers, then it's unlikely that a problem in the wiring in or leading to both drivers occurred simultaneously. If it was one, I'd suspect a loose wire. Since it isn't, then yes, the low frequency section of the crossover is the likely suspect.
Edit: Reading your follow-up comment about changing the crossover to 80Hz, that would just seem to indicate that having the M80 play less bass(and the sub more)made a problem in the M80 more apparent. Try 150Hz to hear what happens.
