Your latest post partially clarifies the situation. The SB-100 apparently has dual voice coils and if you connect two(not one)main channels to the M80s "through" the SB-100 this would create a parallel connection between the M80 and SB-100 voice coils. The net impedance, using the nominal 8 ohm and 4 ohm impedance ratings, would be 1/8+1/4=1/X, X=2.67 ohms.
It's unknown(unless you have the SB-100 manual or other info)if there would be a high-pass filter on the M80s if they're connected through the SB-100. For example, if there was a 100Hz high-pass on the SB-100 outputs, the 4 M80 bass drivers would be rolled off below 100Hz and the net effect with only the single SB-100 driver covering those frequencies would be less bass than with the M80s alone.
