My spouse is a PhD that works with brains (while they are still in people's heads mind you) We all create neural pathways within our brains that affect our perceptions. Each one of us is therefore unique. We simply will never perceive something, visual, auditory etc. the same way. That is why we cannot agree on "what's best" or what is the best method for determining "best". Measuring audio equipment performance is a scientific analysis but becomes irrelevant when introduced to our unique brains while listening.
As far as hearing the difference between amplifiers, I can definitely hear a difference. In my main system I owned an SS amp from a particular manufacturer that was pure class A. It sounded great but at higher SPL's would go into clipping. I decided to sell it and purchased a class A/B amp from the same manufacturer. It offers a bit more power but also offers a much deeper dynamic range than the pure class A unit. Likewise in my second system I sometimes switch between a low power tube amp and a SS integrated. I can tell the difference immediately.
By the way, I can't state this for fact because I am certainly not a scientist but genetics may play a role as well. Think about the unique situation with cilantro. Some people love it while others that have a particular gene perceive it as having a soapy taste. This is my situation. While my spouse loves it I find it completely terrible! Can genetics also affect our hearing? I can't say for sure but I would bet that it can.