10dB resonant peak at 40Hz

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.

Put a pair of subs ([1/4, 0], [3/4,1]) [lengthwise,widthwise] locations on either side wall. There are other locations as well ([0,0], [1,1]) ([1/4, 1/4], [3/4, 3/4]) etc depending on what is practical for you..but you may loose perceivable fidelity with corner loading. Ensure both subs have ’variable phase control’ and adjust it to get rid of these modal nulls, peaks, and more.

Modal peaks will ’linger’ and kill all your room resolution. This is an example of where the avg hometheater enthusiast dude and his myriad of tools these days tend to have many advantages over the old school purist hifi no-sub club. 

Good luck.

@cdc  Subwoofers are not just for adding some "oomph" on the low end. They are "active" room treatment devices for treating/removing modal peaks and nulls, i.e. creation of uniform bass. "Full range" speakers that have significant output below 30 hz will cause all kinds of modal peaks and nulls. When a guy paid 300k for his full range Magico, for example, he will hear a lot of modal issues in his room. Subs are a solution to fix/ REMOVE such modal peaks/nulls.

It will take an enormous volume of passive room treatment (diaphragmatic absorption, etc) to try and address room modal issues at 100hz and below, impractical for a small room like yours, i.e., It will eat up your room volume. Hence, the alternative is to use subwoofers, which are "active" room modal treatment devices. The subs themselves don’t have to be enormous. It can even be tiny lil subs like the kef kc62 that can a)extend down to 15hz and operate up to 100hz, 120hz, etc & b) as long as they are positioned correctly and phase integrated.