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They probably really are 'hearing' a change, though. Presumably every change we think we hear could be a similar figment of our imaginations. The usual wise words would be to say: "Without a blind test we will never know", but I don't think a blind test would settle it either. Sometimes we just have to do what we think is sensible, accept the result without analysing it too much, then worry about it no more.
Looking at the pictures of the system. With such an asymmetrical environment loading the two speakers it is no wonder he has to resort to magic to make the sound tolerable.