I could see how plugging the port could make the mid-range sound better on a front-ported speaker that didn't have mid-range enclosures or an anti - standing wave cabinet design. That's not the M80 however.

Plugging the ports may reduce low frequency distortion and make the bass sound tighter. It might be worth a try if you play at high sound pressures.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated