Sorry, I disagree with the OP's premise. I have multiple combo amps and amp heads now sitting stacked and unused in a spare room that I went through before I discovered a head and cab combo that was "just right" for me and my situation. You can't tell from in-store demos.
Same for me. While the OP may be technically correct that there are bunch of different amps that might sound more or less interchangeable and serviceable in a live setting for one particular player, in my experience finding amp that that sounds "great" in live settings for that one player takes a lot of effort but is possible and well worthwhile.