Jack, I don't think that we can really figure the M22 slope to be that steep from those numbers. First, they're under anechoic conditions which don't take into account room reinforcement. For example, my M22 placement results in essentially flat response at 60Hz, slight drop at 50Hz and much weaker, although still usable, response at 40Hz(essentially nothing in the low 30s). The anechoic +/- 3dB would imply a 6dB drop from a high point at 60Hz and the +3/-9 would imply a 12dB drop at 50Hz, so 6dB farther down than at 60Hz.
The THX ideal is to use a speaker with a "natural" 12dB octave slope below 80Hz. The crossover, which is supposed to be 12dB/octave high-pass, would then result in a 24db/octave mains rolloff and the low-pass crossover to the sub, which is supposed to be 24db/octave, would then theoretically result in a matching 24dB/octave slope for the sub.
