Acoustic treatment

Huh? OK, I’ll repeat it. Maybe it will sink in this time. The frequency of operation for a Helmholtz resonator is a function of 3 variables - volume, nozzle diameter and nozzle length. Therefore, you would be able, well, maybe not you personally, but someone who knew what he was doing, to construct a Helmholtz resonator for 20 Hz that did not (rpt not) have a very large volume. You can also affect bass performance with tiny little bowl resonators. There’s lots of ways to skin a cat.

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.