Rob, looking at your pictures, one suggestion is to have the distances from the M22 mid-woofer cones to the three nearest room surfaces(floor, front wall, side wall)as unequal as possible. The distances are hard to judge from the pictures, but possibly moving the M22s back to just a few inches from the wall behind could be tried.
As to sub boominess, is it simply a bit too loud in relation to the M22s or have you calibrated its level with the help of an SPL meter? Again, trying the M22s closer to the wall behind should also increase their bass in relation to the sub.
