>>The only thing of interest to me here is his claim that the M80's do drop to 2.4 ohms from 10-20hz.

Doesn't the impedence drop to zero (ie just the DC resistance of the voice coils) if you take the frequency low enough ?

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