Peter, the one certain thing I have found from my visits is there is no rule for treatment or toe-in. It is too case by case. Sure, it is easy to make a generalization that overdamped is bad and over live is bad too. That doesn't really help. If an owner is listening in a dead room, he is not a reference but a lost data point.
Apart from that, there is no rule. There are some broad guideline. A lot of videos I post are SETs horns. These are easier to sound right in untreated natural living rooms provided sufficient space. The reason I like them is once you get the speaker right the rest is easier to get a good sound. There are tons of SETs and low watt push pulls that sound good on them. They can sound good wide and toed-in or narrow and straight. Generally, over 4m space is required between listener and horn for drivers to integrate. Preferably more, and they sound better with more space
Panels usually sound better straight. Logans can be toed in or straight. I prefer them in longer rooms, close to side walls is not an issue but they should be wide enough from each other to disappear. I usually find myself beyond the equilateral triangle point for panels, so longer back. I hate the front wall damped on panels.
For YG, Mike's direction, etc, things can sound pretty bad till they sound right. The effort and cost in amplification is far higher. Also, very few have the ability to roll boulder, dagostino, balabo, technical brain, audionet Heisenberg, top Dartzeel etc to find the right choice. For such systems the rules are tougher and they interact very differently with rooms, causing bass boom, brightness, all that needs to be balanced more. That's why treating them correctly becomes more important. Their dispersion characteristic is very different to horns. Needless to say I have found very few good rooms of such types, as you know from my posts. In the smaller/average sized rooms, the best room I heard was Jazzhead's, it was 25ft x 11 ft, so very narrow, but he had partial treatment.
I have experimented with SMT wings at Elberoth, Jazzhead, Flyer, been in 3 full SMTed rooms, been in many GIK type panel rooms, and my Avalon friend in London has a great room with a couple of side diffusors and a second room that he partially treats. Bill with his Focal used to experiment with moving absorbers around which I was exposed to. I myself had GIK traps which I sold off immediately because I did not like them, they killed sound. I also went from a barren floor to full carpeted which while better, I should have kept partially carpeted. Mistake. Mike's and Marty's when he had the pipedreams were the best rooms I heard in terms of infinite headroom as if you are in the open without noise of the street. Comparatively, with everyone else you know you are in their room. Mike also has the lowest noise floor I have ever heard, so that helps.
But no, I don't know which treatment works best and is a general rule. For my chosen direction, it will be SETs horns in a living room with no planned treatment, which I am happy to change to minimal treatment. Also most people with normal rooms get a speaker too big for their rooms and simple Tannoys, devore orangutans, etc sound much better than an incorrect speaker
Also realism factors for SETs horns to sound real and for SS Cones to sound real are very different. People ask me all the time how I can like Yamamura and Pnoe Mayer, and also Mike's system. My answer to them is they are different schools and I am not stubborn
And yes, just paying someone to set up a room and treat it doesn't work there has to be trial and error adjustment after that
