If distortion was the only thing that mattered the nut would have been cracked long ago. Many things can go wrong within an amplifier design that reduce the musicality. Some are hard to test for, even using a null test. Micro instabilities can create havoc, but being very small and only occationally present are very hard to measure. If they only occur when a loud symbol is played and then only over a cirtain volume range, it would be hard to detect with electronic measurement devices but the ear can detect the graininess. Although uncalibrated the ear-brain is still a very good measuring instrument.

Hifi is about creating an illusion, the re-performance is never actually in your listening room, the Hifi needs to fool the brain into thinking it is. Good hifi therfore has to be better at convincing you that something unreal is happening. Psychology has a lot to do with it.

Objectivism: Letting ones brain fool the brain.

Subjectivism: Letting ones ears fool the brain.

Psychological profile will probably have a lot to do with wich camp you follow.

For myself I'm just a raving loony so have no strong feelings about it. When I design equipment I use both methods, I do prefere the subjective part most, but who wouldn't enjoy sitting down listening to music more than slaving over a hot oscilloscope.

Enjoy the music, not the hifi.

Chris