Curiously, I can hear these differences better when I turn out the
lights and allow my hearing full access. Perhaps those who claim
'sighted' tests are invalid are picking the wrong reason, if they
claim that 'knowing' which amp is being tested causes bias. The
simple fact is that listening in a lighted room causes your brain to
expend energy on vision and have less 'processing power' left for
hearing.

Is that a fact? It's wonder they don't douse the lights completely at
concerts, then.

Not if you realize that concerts serve an experience that includes seeing
musicians perform. Seeing musicians perform allows one to appreciate a
performance that hearing it alone, no matter how clearly it is heard that way,
does not allow. Keep the lights on at concerts please. Even if it makes for a
less pure listening experience