John Galt,
The phenomenon you are referring to of the sub feeding back to an M80 is in fact possible with any sub-woofer / front channel combination that are both capable of reproducing very low frequencies. In order for this to occur you must be using the high level input on the sub. You would be using the sub output from your receiver so this could not occur. What happens is if someone were to say slam a door hard enough to move air in the room and thereby cause the M80 driver to physically move this movement would create a signal out the speaker input (a motor is a generator when physically moved). This signal would be transmitted to the sub and the sub would reproduce it. The air moved by the sub would then physically move the M80 woofer again if they were very close to each other and around the circle we go.
