10dB resonant peak at 40Hz

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.

My room is 13 x 15. I can try to single sub aimed at the wall.

Does it help if the speakers do not go below 70Hz?

@deep_333 

Modal peaks will ’linger’ and kill all your room resolution.

Not clear what you mean. The 2 subs will produce this? Timing is important to me. If they can not integrate properly might as well forget it.

The subs have variable phase control. Are 2 subs enough? Not real clear about placement.You mean 1st sub 1/4 of the length and next to he wall on the width? The second sub on opposite wall  placed 3/4 of the length? Which way do they aim?

I think I was missing the point. You are using a subwoofer to break up the 40Hz standing wave produced by the loudspeaker? Not to produce bass itself?

@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. 

You are using a subwoofer to break up the 40Hz standing wave produced by the loudspeaker? Not to produce bass itself?

@cdc Yes.