The minimum distance would be the diameter of the rear ports. This is to keep the air in the port from compressing and changing the response of the speaker. After that, it is up to you.
You will get more bass reinforcement the closer you get to the wall. If you are using the M80s with a sub with a crossover set in the 80Hz region I suspect that will counteract any bass reinforcement.
Short answer, about 4" from any structure behind it.
Are you putting it inside a stand? If so, is the stand open at the back?
