The phenomina you are describing is correctly referred to as "1/4 wave cancellation" in acoustic circles. Your understanding as it occurs on the front wall is accurate as described. The 1/4 wave cancellation also applies to sound waves reflected off the back wall to the listening position. So in your example, if the listening position is 5 ft off the back wall, you can expect another cancellation at 56 hz.
Anything you can do to absorb the incident wave on the wall at the cancellation frequency will help. Of course, at these fairly low frequencies that can be a challenge. Bass traps, even 20" in diameter do not absorb much at 50-60 hz. I believe a Real Trap is also ineffective at this frequency.
One solution is a helmholtz resonator which can be tuned to a specific frequency. F. Dalton Everest in "Master Handbook of Acoustics" has quite a good discussion of how to do this. They can be boxes, corner traps or policylindrical absorbers. If you don't have this reference it would be worth while to buy one (new or used on Amazon).
I built a helmholtz resonator behind the rear wall of my room with good success (see my system). Of course, I was starting from scratch and you may not have that option. I also built in corner traps, however, while they help reduce excess low frequency energy, these are primarily aimed at reducing the RT60 in the room.
You also need to look at the natural room modes and take into account their interaction with the speaker location. A simple free tool like Room Response Calculator, by Yavuz Aksan, is quite useful for this purpose.
I am curious how you did the measurements to confirm the cancellation? Remember, you are measuring the accumulation of all effects, not just 1/4 wave off the front wall.
