I agree, It is why I mentioned potentiometer. Most preamplifiers and receivers use a mechanical Potentiometer with two resistive
tracks and a mechanical wiper that follows the track. If your idea is correct there are decades of equipment that adversely effected the reproduced sound. The truth is that you room and listening position have as much or greater effect than this.
As for testing, if you are in control of the test there is very little validity in the results. There is a very real effect called confirmation bias. It is why ABX testing was invented.