The impossibility of sound quality difference whatever the medium

[iamwappie]

Don't forget the placebo effect.
The placebo effect is real and works.
In the case of curing a real illness only some illnesses can be cured by placebo cures in many, but not all, individuals. Surprisingly (to me anyway) this is the case even when the subject knows they are receiving a placebo. It is a sufficiently well known phenomenon that in some countries placebos are a permitted medicine for some ailments.

If the subject believes the placebo to be expensive it is more effective.

My personal opinion is that whilst many people hear an improvement due to cables there is absolutely no reason why a real one should actually exist between adequate cables and the difference heard is real to the listener but is simply a manifestation of the placebo effect.

To rule out a placebo effect there are blind tests.