A Placebo LIKE effect in audio, is meant to describe a similar situation to the medical scenario of, when someone knows something has been altered, changed, improved, or adjusted, they hear a difference.
Often even when removing the difference or adjustment, and the person is not aware of the change back, they still perceive a difference.
It is a very well known effect. Same as how mood affects how a system can sound at times, or when one is tired it sounds more strident etc.
Our brain is in control, not the ears.
Often spending a large sum of money on something, will release chemicals in the brain, and create a mood that facilitates one almost "Hearing what they expect or hope to hear", whether true or not.
Kinda like how when you first drive off the lot with a new car. The car seems so fantastic, but a few weeks or months later.....kinda just normal.