Alan has pointed out once or twice that the "midrange dip" in the M3 family of speakers is related to weaker off-axis response in that frequency range from the 6-1/2" woofer relative to the 5-1/2".

It might be interesting to do a "sibilance hunt" between the 5.5" mid family (M2,22, 60,80) and the 6.5" mid family (M3, 40, 50) to see how they compare on and off axis. If only we had someone with both speaker families and a bit of time...

Chess ?

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8