...A hardcore subjectivist claims all amps sound different even if they measure the same, and that is based on hearing alone, and only the proof of a personal opinion which we know is unassailable...
But, is it?
A friend of mine, Dave Slagel, winds transformers. On a trip to the Beltway, he was asked if he listened to the direction of the wire before he started winding. "Of course, not." His response was challenged with a test. He would send samples where only he knew the direction. He took them up on the bet, and wound several small spools in one direction, and a remaining one in the other direction. All this wire came from a large spool, and very fine wire was used, so that visual cheating was impossible. Then, he sent all the spools to a neutral supervisor. Later, they were returned with notations indicating the direction of each wire, and they were all correctly marked. I cannot hear the direction of wire, and neither can Dave, but we accept that some people can.
Some people don't believe in stylus drag, even when empirical evidence exists. All this tells me that our measuring devices aren't as advanced as our ears, or we are measuring the wrong things some of the time.
Steinway & Sons has three identical listening rooms for a reason. They realize each piano has its own character. What a lot of people in audio don't realize is that our equipment are instruments, too. No one can truly define neutral.
...A hardcore objectivists claims all amps that measure the same (and this means more than just THD or driving a standard resistive load...because a technical ojectivist knows electronics and the interactions of circuits) will sound the same, and that is based on repeatable universally understood measurements all traced back to a world calibration standard...
If that's the case, why didn't we keep all those old Japanese receivers from the Seventies? They measured great!
Summary: Measurements are a valuable point of departure, but it doesn't stop there. If it did, audio would have been perfected long ago, and we would all have the same system.