Maks sure the straps between woofer and mid/tweeter are tight
I think John had it nailed though... you're sitting in a null for at least two dimensions (depth and height) because you're "half a room" away from the speakers depth-wise and "half a room" away from the woofers height-wise. The easy fix would probably be to put the M80s on the long wall, which would fix the "half a room" depth problem *and* eliminate having the same distance from both back and side walls.
If rotating the room isn't an easy option, then try :
- moving the speakers a couple of feet closer together
- raise the M80s a bit to get the woofers and ports away from the ground; alternatively, shuffle the furniture around to put something relatively low in the main listening position
I won't even joke about getting a sub, but this is one of those cases where you don't want the bass driver to be located in the place you happen to want to put your mains.
