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.

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". 

@cdc What are you using to verify that it was 40hz? Do you have an HVAC system near the room? As silly it my seems,  I have a 30hz peak in my room and it is similar in size so every time the AC unit is on which is right above the room, presents rumble at that frequency.  I use a RTA to measure room response every now and then when I change out speakers or subs for that matter. 

Another situation is consistent of balance of frequency response of a bass guitar or any matter of instruments that has frequency response that low. 

Yes, DSP would do it. I'm not using a sub.

@cdc You have a standing wave that is causing the peak. If you poke around, I think you'll find you also have some dips caused by the same issue.

The best way to break up standing waves is by using subs placed asymmetrically about the room. They must not be active above about 80Hz else they will attract attention to themselves. Because 80Hz has a 14 foot waveform (so 40Hz is 28 feet) the bass in your room is 100% reverberant before your ears can make out what bass note it being played. For that reason, the subs can be driven with a mono signal. 

Bass traps help, DSP helps (and can be pretty good about peaks but can't do anything about dips since they are due to cancellation) but both techniques together offer about a 5% solution if you have a dip. Subs offer about 95% effectiveness since they can break up the standing waves causing the problem; they work so well you may not need any other help.

This approach to dealing with standing waves is known as a Distributed Bass Array. If your main speakers make decent bass, you really only need two subs to sort things out.

@atmasphere 

My single sub with the HP filter is doing such a nice job, I'm considering adding a second. But I'm concerned that while two may fill the room more evenly, I'm inviting phase issues with two sources. I'm thinking that one reason a single sub sounds so good is the absence of two sources of low end interacting with each other and the room. 

I've already found that adjusting the phase on my SVS SB1000 Pro has a significant effect on the overall balance but with two it could get much more complicated. 

Your thoughts?