Picasso and Matisse are demonstrably different. Amplifiers that have achieved audible transparency are demonstrably the same. Comparing two identical things and determining them to be identical does not require any subjective interpretation. It either is, or it isn't.
As I said, I test every piece of equipment that I buy. Over the past thirty years, I've tested every amp I've owned, and I've even compared them to my brother's system. Back in the 70s, my family had a Heathkit amp that hissed like a radiator. But since the late 70s, I haven't run across a single amp that sounds different. I've compared Sanyo A line, McIntosh, Sony and Yamaha. All were audibly identical. What went in was what came out the same except louder. That's exactly what I want in an amp, and doing a comparison test to determine that isn't at all subjective. All it takes is a switch box. (Yes, the amount of time between samples matters. Audio memory for two similar sounds lasts no more than three seconds. A gap larger than that and you are inviting bias into your results.)