Good question - as in acute.
The reality - objective reality - that I perceive is that for some reason - non measurable - some stereo kit just sounds really inviting - warm (now measure warmth), entrancing to the human spirit - at least to this one. Probably much the way some musical genres simply sound better than others. I like some 70's pop - and Rachmaninoff - Second Symphony in particular. And Saint Saen (organ symphony) and Debussy (la mer) and Philip Glass. Those tunes may leave others completely cold - fine - but try and measure something objective about them. Impossible. Hey track 5 on Led Zep 2 really does it for me - as does Dazed and Confused.
Thousands of tunes light me up - and tens of thousands of offerings leave me cold. So how can that be measured?
Same with stereo gears - some of it - and some of that very expensive - just leaves me feeling that there is "nothing there". Same with digital - many digital releases - particularly early ones - are simply unlistenable. Same with MP-3 and most other compressed music - it just seems that the music is missing - though the sound is still there. Perhaps that can be measured.
Want an objective example? Here is an F/R plot of a Martin-Logan ESL-13A - looks like trash. Toole addressed these failings as "resonances" - I think they are drum modes typical of all membrane speakers. But look at that plot;
BUT - and it is a big butt - I have heard those speakers on a friend's rig - and they sound superb. Just wonderful. So what is wrong here? How does the terrible objective measurement not reflected in the actual sound of the speaker? Toole's plots of the same speaker look even more trashy - and Toole was dismissive of that particular speaker. Fine - but it still sounds inviting and entertaining.
Same with my Darling Dynas - those things have enough self noise for about six high end stereos (I jest) - but I am sure that they measure terribly - old valve amps, creaking along 50 or 60 years old. They hum, pop and generally I am sure measure inferior to almost anything. But they sound so good - so warm, so inviting, so musical - the timbres, the tonality, the dynamic contrasts? Magnificent.
So now you are left with only one thing - declaring myself deaf and possessed of no taste. About what I would expect in any kind of argument with a stereo zealot - no matter their particular religious bent.