Ash, welcome. If the room dimensions that you described are accurate there should be no room modes between 32-40Hz, but a very strong one around 43Hz. This would be caused by the identical 13' height and width dimension(coincidentally, on a thread yesterday we discussed a bass problem with Mark Johnson relating to his 13' square room)causing a double reinforcement at that frequency(560/13~43). The speaker placement to reduce the problem isn't especially practical. The woofer cones would have to be raised to about 6-7' from the floor, where the 13' floor/ceiling mode is at a minimum. Alternatively the speakers would have to pulled out from the 21' wall so that the woofer cones(not the back of the M80s)are about 6-7' from the wall, where the 13' width mode is at a minimum. You might want to try the latter move just as a temporary experiment to see what the effect would be.
If speaker placements(or lowering the ceiling)aren't practical, equalization with e.g., BFD is the remaining possibility. Different speakers or a separate sub(assuming that they have strong normal response at that frequency)should have a similar problem, so it's unclear why your other speakers didn't exhibit it.
