Thanks great post!
But there is 2 part in acoustic management for a small dedicated room...
1) Balancing diffusion/reflection/ and absorption is conventional passive materials treatment..
2) active mechanical control with Helmholtz resonators and diffusers are the missing part of your acoustic description...
And you mix together many things that perhaps dont work with some which work... I own a grid of cheap Schuman generators i myself modified and it work...But the audible effect, at the same time subtle and powerful, could be hidden by a not so much refine room or a problematic system, for example with speakers not isolated mechanically from vibrations or in a house with a too high electrical noise floor ....
In the race to reach a good S.Q. people rush to EASY solutions, and some begin to try Shuman generators but it is not this that we must begin with in the first place....It is way better to eliminate vibrations from the speakers for example....or like you justly said manage material acoustic passive treatment...
The problem is people are ready to pay for costlier "tweaks" BEFORE learning the basic acoustic...
My best to you....
When you take the time to learn how audio works, and then instead of wasting your money on things like magic fuses, over priced cable, AC noise isolators that do nothing, Schuman toys, teensy acoustic resonators, and all manor of stuff that does little or nothing, and spend time and money on the stuff that matters oh, like acoustic panels, bass control including sub arrangements, achieving an in-room response that is effective, learning to balance direct/reflective sound, and speakers that don’t distort ... then you may have a system that sounds good.
However, you know you really have reached the pinnacle when the owner of an audiophile tweak company goes out of his way to insult you on audio forums.