I can definitely hear the M80 with a 20Hz tone. With one watt in, I get 66dB at 3 feet (1m) on axis which matches this graph.

By the way, I know I am putting 1W in because I adjusted the voltage at the M80 to be 2V. The graph above tells me that at 20Hz, the impedance is 4Ohms. So the power is (2^2)/4. So this means the M80 meets spec.

And the mids are moving at about half the rate but are in phase with the woofers. So this means the M80 is producing subharmonics (which may not be unusual for a speaker being driven outside its specified range now that I think about it).

I'm not going to worry about this anymore. The M80s shouldn't experience low frequencies such as these in normal listening.