10dB resonant peak at 40Hz

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.