The 80-Hz suggestion for a crossover frequency for the M22s to the subwoofer is a guideline; however, the rationale for that frequency is to avoid a "hole" (suck-out) in the bass response as it transitions from the M22 to the subwoofer. If you look carefully at the NRC measurements of the M22's bass output in the anechoic chamber here. . . http://www.soundstagenetwork.com/measurements/axiom_m22ti_se/
... you'll note that the bass output remains quite linear to 80 Hz then drops dramatically so it's more than 12 dB down at 60 Hz (that means the bass output is about "half as loud" as the output at 80 Hz). By using the 80-Hz setting for the M22s, you can be reasonably certain that your subwoofer's response will be reasonably linear to 80 Hz and higher, hence the slope of the subwoofer's bass output will coincide nicely with the M22's response at 80 Hz.
That's the theory. Of course, if you figure in some room gain and allow for subwoofer placement and standing waves, you may find using a lower or higher crossover frequency for the M22s will work better in your situation.