What makes the biggest difference in sound quality?

I have as well...using a wealth of information synthesized over the years and well documented from various sources.

I agree based on experience its fruitless to focus on gear alone without first a focus on integrating the gear into the room properly, starting with the speakers. Lots of proven references for that and many services available to do it for you right if in doubt, if you can afford it. It has made a night and day difference for me. Acoustics are 90% of the battle.

I would very much like to understand the cited Helmholtz method as it applies to pro or even just home audio in order to be able to assess it and maybe even attempt it if viable but google search turns up nothing.

So how does one practically apply this method? Has anyone documented it so that others could attempt to execute correctly? Is it just trial and error? An Educated guess of some sort? IS it used by professionals? I would expect any robust process would be adapted at least in some cases by professionals in certain applications like studio design, etc. and be well documented in the process.