Sycraft's comments here are right on the money, in my opinion. He's basically saying the exact same thing I am:
Ok, great, however what we empiricists do NOT know is if we have discovered everything that is important to human hearing. We know frequency response is, so we measure that. We know noise is, so we measure that, etc. But, what if there is a property we DON'T measure, beacuse we aren't aware of or believe it is unimportant, that is important to perception? Well that would mean, despite our claims that two things sound the same because they are identicle on the scope, we are wrong.
Also, I think he's got a great point on blind tests:
Build two amps with just razor fine tolerances on everything, and have different opamps in them. Then, put them both in identicle boxes, rigged to be tamper resistant. But a label A on one box, and B on the other, randomize which box is labeled which. Give them to test subjects for a good period of time, a couple days at least. Let them go back and forth all they like. When their time is up, take back the boxes and ask which was better, or no difference and WHY they liked it better.
So, when you get some blind tests that were done that way, then it will really show something. It's really common for some little flaw to creep up after a few days of listening that just makes it fatiguing and unenjoyable, making most types of blind tests regrettably impractical.