You have a beautiful home.
I'd get a test disc and run some sweeps (there are probably a number of places you can download and then burn a CD with test tones). Or find some material that has pretty consistent bass content. Change your position in the room throughout and try to listen for differences. It seems likely to me that your listening position is in some kind of null created by the specific dimensions of your room. i.e. is there a difference in bass response at other positions in the room?
And definitely check to see if all the drivers are firing.
It's kind of hard to see from the picture, but try moving any "stuff" away from the M80's and see if that makes a difference.
I'm not sure what kind of material you are listening to. The M80's should provide quite a bit of impact down to about 40Hz, even in a big room.
We are a whole community of "that guy" - StPatGuy