The hell with test conditions: my point is, can you tell the difference when you
don't know which one is in? Because if you can't, then they indeed sound the same as measurements would suggest, and your psychological bias is the
only thing that makes a difference.
Here's a simple way to check if you have two amps: very carefully match levels with a multimeter, then get a friend and turning your back to him, ask him to unplug the amp and then randomly either swap it or plug the same one back in, and then see if you can tell if it was changed or not. Or, if you want fast switching, then use a rotary selector. Do this ten times or so and then see if your guesses were right sufficiently more than 50% of the time, else it was random guessing. How much is sufficiently more depends on the number of trials.
Some things indeed sound different, but also a lot of differences simply disappear in a proper blind test. And if you can't hear the difference between two things in a system where the rest of components have really high resolution so the maximum differences should be clear, then why not save money and use the cheaper one? Otherwise, it's just a fetishism for fancier gear, simple as that!