What makes the biggest difference in sound quality?

To answer your specific question, in my system, of the items you listed, I believe power cords have the biggest impact on the sound quality of my amplification components.

@ted_denney 

Inference to conclusion will take place in the minds of those reading.

Yup, we agree on that, but you wouldn’t want to know my conclusion.

@mahgister - I thought you might be interested in the statement made by former dealer @richopp in a different thread where he said:

Accuracy in reproduction depends more on YOUR ROOM than any specific gear.  Once you realize that, the rest is just a hobby.