Once I had a problem with my m80's where a wicked vibration would occur at certain frequencies. I swapped speakers and it stayed with the speaker (not the L/R channel) so I thought the problem was the speaker. However, after swapping grills I noticed the problem followed the grill. After closer examination the grill appear to have some wear on one of the grill posts that was not "snapping" in and leading to vibration of the grill against the m80.
Axiom sent me a new grill and I haven't had any problems since.
