10dB resonant peak at 40Hz

@OP - Can you move the speakers closer to the front wall? Even if you still get a resonance you just don't want it on a root note that bass players land on regularly - but even a relatively small movement might tame it anyway.

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.

Dsp is your best option. 40hz is roughly 27’ long so you could put your speakers in the 40hz null which would be 1/4th of 27’ feet… works out to about 7’. But this is only one boundary and all boundary sum…

bass traps would need to be super thick at 40hz. Like 3-5’ feet (just a guess I have no idea). Off loading to subs and using subs to control the room is another option.

Not enough info on the OP, but, I am guessing you are sitting right up against or close to the back wall?

Peaks are easy to handle with dsp (not the case with nulls). If you don’t have dsp, put one sub on front wall and a 2nd sub on the back wall (hopefully you have subs with variable phase?). It should be gone.

You can "remove" peaks and nulls when you have freedom of placement with subs that come with more user dofs. Subs are not just for adding oomph to the low end. Their primary job is to remove peaks/nulls and throw you in a uniform womb of bass (one key ingredient for audio nirvana).

You could also do other discrete tuned resonators, but, you may need to find some nerd to help you with it.

I just set up a sub with a high pass on my mains. It's an astounding and wonderful difference. Just having the low end coming from the floor instead of 30 inches above eliminated most of my 60hz resonance. 

The difference in clarity with the mains freed from low end duty is pretty dramatic. Coupled with clean and even low end it's even better than I expected. 

After playing with the parametric eq, I ended up not using it. Playing with the low pass roll off curve was more helpful in blending the low end.. And variable phase surprisingly making a big difference. Still fine tuning but much closer than ever to "the sound".