I agree with everything you've said. The human bias position is the go-to position I use in these arguments. If bias can steer perception into a state that agrees with our subconscious then it could explain away a great many anecdotal reports. However I would like to try and focus on real changes that may lead one to think that the tone had changed.
Audiophiles could just argue "well how can most people agree on the same things re Yamaha? Mass delusion? Yes, frequency response is flat at 1 kHz, on the test bench into resistive loads, but what about reactive loads from 20-20kHz?"
The way that I see it, people can experience audible differences in three major ways - 1. either via engineering real audible differences due to lopsided comparisons which ensures non-parity (ie listening at different levels, slow-switching, sighted bias ) 2. imagining differences not inherent in the equipment at all, due to the placebo effect 3. experiencing real audible differences that are really false positive outcomes (eg comb-filtering artifacts by shifting positions on the couch, even by 2-4 inches, may lead to subtle audible differences.
Granted, placebo effect can never be discounted here, but I suspect that people experience changes that are self-made. I think in many cases the differences are real and there are real physical explanations for it, it's not all imagined in the subconscious.I do think the relative gain and volume pot argument is a credible one which could explain ones observations.