I suspect on the 788 you will just have one option for all speakers regarding the crossover point. The 788 owners manual will provide you with such information. If this is the case a crossover point of 80hz all around will be a very good starting point for your mains, center channel and surrounds.
Regarding the crossover point selection on the M80's you will hear of 40hz, 60hz and 80hz. Alan Lofft of Axiom has said in the past he likes 80hz, on the other hand the lead engineer and owner of Axiom likes the 40hz crossover point with the M80's. There are also some that like it set to 60hz.
Point being, play around with crossover implemantation yourself between your mains and sub and see what sounds best in your room.
