"Unfortunately, some of the members of this forum, while intellectually
understanding it, have a difficult time differentiating between "sight
*may*
provide a bias that overrides true differences" with "sight *always*
overrides true differences and makes your comparison invalid". They
should
know better, but they don't seem to be able to allow even the possibility
that there are real differences and that you might have heard them."

We don't prove a hypothesis, we fail to unprove it,ie. after time and
enough effort has been put into a hypothesis and it continues to be
unsupported, we turn to more fruitfull lines of questions. The above
hypothesis is one such. After decades of tests in humans the idea of
there being no expectation bias has failed to be supported. The
continuing hope that one more test will suddenly confirm there is no such
bias is very slim and we can't really put any faith into it. But if one
insists, it would be an easy test to do it once again with amps in a
structured blind test in the hopes that finally results will tend away
from random.