Can you elaborate how you find the positions for the speakers and the listening chair?
Do you walk around the room and listen or do you start systematically and then fine tune in an iterative process?
Is your approach similar to the "focal point" method?
Wilson Audio advocates their WASP technique. As for changes of 12-18" those are huge. When I set up my speakers I was moving them in 1/4-1/2" increments. I have posted here that a 1/4" toeout of the speaker resulted in an 8" change at each ear
The beauty of my carpet is that it served as a grid with the darker horizontal lines being 4" apart from one another By moving the chair forward as I did served to remove the room from my equation and the change in chair for me was a wonderful revelation as the larger chair I previously used was just to tall, too deep and too wide. With the new chair it is as if it is non existent as I hear no chair reflections especially from the top of the chair near my ears. It was as if there was suddenly more music and heard with much more presence and a feeling that I am totally immersed in the sound.
I am still shaking my head about the speaker position and toe in that should be done but in spite of this I love what I am hearing. More of a "you are there" experience
If there is a downside to the whole thing, it is minor and will just take some getting used to.....in the old sitting position, where I was seated, I could easily bend to the side and adjust left and right channel volume (Lamm does not believe in remote controls) very easily. Now I am a few feet further ahead of my preamp and any volume change necessitates my having to get out of the chair to adjust the volume