I'm curious if any instruments were used along the way during the setup process or if its all being done by ear? Maybe something as simple as an SPL meter or maybe an SPL meter with signal sweeps being played back so you could see/confirm changes taking place?
I've witnessed in my own room where one speaker was almost 8db louder than the other speaker was after a better than average attempt was made during initial setup. At the time this was done using only tape measures, laser pointers, levels..etc. But after some time had passed I would start noticing that the sound stage and output level seemed a bit more pronounced in that Right channel compared to the Left one. Once I broke down and pulled out my calibrated SPL meter and measured it, the problem became quite clear but without the Meter I couldn't really be sure what was going on.
In any case, the fix was a very small movement of one of the speakers that was measuring almost 8db louder. I would consider this a small movement but it was more than 1/16th. Maybe an 1/8-1/4" of the whole speaker itself... to the Left.
My assumption was that the ceiling peak on that side of the room that didn't exist on the other side of the room was a contributing factor and the small movement kicked the speaker out just enough to leave that gain zone it was apparently sitting in.
I guess my point being, I don't doubt that very small movements can make a difference but its hard to imagine how someone can really tell what is going on unless, at minimum, an SPL meter is being used along the way.
I cant say for sure if an SPL meter was being used in the case of this post or not and I'm certainly not suggesting that one wasn't used. I only bring this up because it wasn't clear how these very small movements (1/16th) were being confirmed outside of someone's hearing capabilities.