Acoustic treatment

Sorry folks. It falls off like a rock at 18 Hz. My typing is worse than awful.
They did not teach boys how to type because we did not become secretaries. 

Geoffkait. Still not a direct answer. I probably knew what a Helmholtz resonator was before you were born. In 1978 I had two RH Labs (Randy Hooker) sub woofers. They were essentially enclosed Helmholtz resonators adjusted to 30 Hz if I remember correctly. They were by today's standards huge. I do not have a scientific calculator but to make something the shape of a tube trap a 20 hz Helmholtz resonator it would have to be huge. Think of a 20 Hz organ pipe. 32 feet long. Yes I know it is a different principle. Oh, and the equation is

                          F = v/2 pi times the square root of A/V L

F is the frequency, v is the speed of sound, I hope you know what pi is,
A is the cross sectional area of the stem, V is the volume of the chamber and L is the length of the stem.

I must have gone farther in math than you did. No offense,

The reason I ask about adding smaller subs to a big one is that I, like a lot of people I'm sure, already have one big expensive sub.  I would never buy three more like it.  I measured the response with the Behringer ultracurve and measurement mic, and equalized out the worst of the problems.  I'm not unhappy with it how it is but if I could add a few smaller, cheaper subs and get a dramatic improvement I might consider it.  

Deeply discounted subs are common.  For example, I could get three KEF Q400s for about a grand.  It's a sealed box like my current sub so I'd expect them to blend fairly easily.  Adding three more drivers would allow me to let the big sub take it easier.  The question is whether adding fairly cheap subs would solve more problems than it causes.  Would they start audibly distorting if I turn the volume up a bit or would the fact that there are three of them plus the big one allow for more clean dynamic range than I have now?  Maybe I'd leave them off if I want to listen loud and only use them to create really high quality bass when I'm listening at moderate volumes.  

Here is a good site with various materials and their effectiveness
https://www.bobgolds.com/AbsorptionCoefficients.htm

I have had good results with some bass absorbing materials, multiple subwoofers and using the free software REW and MSO (Multi-sub Optimiser). 

Most anything big and absorbing can work. I have a wall to wall closet stuffed with pillows, sleeping bags, boxes of clothes etc.

Working with fiberglass is not bad unless you cut it. You can wrap 703 in muslin (cheap cotton fabric) before working with it or just leave in the box and throw the whole sealed box in the corner. Wrap it if you like. Aesthetics is another issue. Thinner panels only do so much