In 2 channel mode on the 3808 you have to set the 2 channel speaker settings and in there you have to allow the sub to be LFE + Mains, if you desire to have the M80s as large otherwise set them to small then the LFE + Mains setting is moot as the receiver will default to sending the frequencies lower than the crossover to the sub regadless.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70