I also asked them in their support center and this is their reply today.
Provided that they are (as a pair) well
ventilated, than you shouldn't have a problem stacking
them. Issues arise when overall ventilation is poor
and highly demanding speakers are played at
"excessive" volumes.
I have them on the bottom shelf of a flat panel stand that is open so even though I intend to crank M80s up excessively I think I will be ok