10dB resonant peak at 40Hz

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

@cdc Yes.

My personal conclusion: this is a room, not a speaker problem. There is nothing wrong with the speakers per say.

@cdc There's no point to bass traps and/or room correction until you have the standing wave issue solved. That will take care of 95% of the problem and the former two the remaining 5%.

So yes, a DSP is really the only solution.

Now if you only have one sub available, one way to put it to greater effect is to aim it at an angle towards a wall, such that the bass wave bounces like a cue ball before getting to your ears. I've found its pretty effective at killing standing waves. Often you are looking at a corner of the subwoofer cabinet but if you can manage that it works a treat.

The other thing to keep in mind in your situation is you don't need 4 subs to execute a Distributed Bass array, since your main speakers apparently do alright with bass. You only need two at the most. But try my suggestion above and see what you think. 

Your thoughts?  

@mashif 'It depends'. If the subs are only active below 80Hz than interactions won't be a problem unless you place them side by side and operate them out of phase.

If you have only one sub (and its only active below 80) then its a matter of placement; you have to move it around bit by bit until you find a spot (which you may already have found) where all frequencies can be heard at the listening position. Two subs makes that go a bit easier; 3 even more so and you really don't need to go past 4.

My room has a pretty bad bass cancellation at the listening position and my speakers are flat to 20Hz. To break up the standing wave causing the problem I added two subs- one to the left of the listening position and the other to the rear and a bit to the right. It was pretty easy dialing them in.

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.