Many moons ago, I came up with a simple demonstration
that showed clearly how a number of different make and
model stereo power amplifiers sounded exactly the same.

"Sounded exactly the same" is not the same as "were indistinguishable under
the test conditions".

With respect to context, you stated that the purpose of the test was to
convince audiophiles that their belief that every amplifier had a distinct
sonic signature was invalid.