It’s not even placebo...contextual stimuli actually will change how things sound. If you have decent speakers at your computer, listen to a passage with your monitor on, then the same passage with it off, it will sound quite different.

Another example from audio production, adjusting signal processors while processing is bypassed, and being convinced the sound is changed.

Another one I’ve been noticing is how the resolution of the gain control affects perception of the “power” of the amp. If the resolution of the volume knob is high, meaning it takes more turning motion for the same level increase, the amp “feels” weaker to me, biasing my perception of sound.

The mother of all dilemmas when it comes to comparing audio gear is that the louder source always sounds “better.” This a very powerful perceptual distortion.

So while I think stereotypical audiophile practices are non-sensical, I have no doubt that they really make the systems sound better to the practitioners!