I agree with the others here, and while M80s are great, you still are missing the ultra low effects for movies. There are a number of sound mixers (as in the guys that do the mixing) that have been adding effects below 20Hz (some are below 10Hz - eek) that the M80s won't give you the impact that they are trying to get you to experience.
Heck, my wife, who doesn't like movies played too loud, like our properly calibrated subwoofer effects. I say that because way too many people crank up the sub way too loud. With that said, I have been using a big sub in a 14' x 24' room for a while, and then I added a second large sub. Not to make it louder, but to get even better (smoother and less boomy) bass response in the room. Both are only set to about 25% of their gain (sort of like volume, although you probably already knew that). Yes, I can crank it up until the fillings shake out of your teeth, but that is dumb.
