10dB resonant peak at 40Hz

Second, I have not had good success with DSP as it changes the FR but also degrades the sound quality.  Third reducing sound at 40Hz does not address the problem. Sure the hump is gone but I think I would loose bass quality as what is left is still a resonance and not music.

What did you use for DSP?

Your concern about hearing resonance rather than music can be addressed by a combination of HP filter and sub. By reducing the energy below 60hz in your mains and REPLACING it with a sub with DSP that you can move around, you would have some control over the low end. The SVS subs have great DSP software that can tailor the FR precisely in the low end without affecting the overall SQ of your mains. 

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.

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.

I have not had much success with DSP or room treatments. I think the best solution is get 2 subs but where do I put them?

My current speakers only go to about 70hz so low bass will only be from the subs. I was looking two The Cube Active Acoustic Suspension Subwoofer but open to other suggestions.

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.