You asked : From most distortion to least distortion, which system component contributes to audible distortion at the seated position?
How can we conclude from this poll that "those interested in best sound quality should focus their efforts on the low hanging fruit in their systems. Rooms and Speakers is where the biggest gains will be achieved in all systems. "?
Also IMHO your conclusion completely ignores the accommodation factor - although our room and speakers introduce high "distortions" (in your sense of the word), the brain ignores them. And still it is very sensitive to other type of minimal "distortions". Happily this happens - there is a strong disagreement about room treatment and what is the best speaker type between the audio experts.
BTW, most of us know that a statement carrying the word all is a risky thing in sound reproduction.
Here is an interesting read.
http://www.soundstagexperience.com/...ono?utm_source=twitterfeed&utm_medium=twitter