Another thought not suggested above -- add a high pass filter to the existing speakers and add a powered sub or two. You can try the sub in different positions to find the best spot to even out the 40 Hz resonance. Subs can often be easier to move into different spots without bothering the other uses of the room as much.
As an additional observation, I've noticed that sometimes the placement of the main speakers which gives the best imaging and soundstage may not work as well for bass resonances, and vice versa. Since most of us don't have dedicated sound rooms, this means we have to co-exist with home decor and such. Using a sub can be a good way to handle things.