My point is very simple: excellent sound component manufacturers know how to test all components, including parts used in product’s assembly, and final product test, in production flow to ensure every unit “meets the spec”, and therefore they have in place all needed test procedures, equipped with accurate test equipment and provide training for staff.For this to be true these manufacturers would need specifications that include everything that affects how something sounds.
They do know how to make sure their units meet their specifications.
They do not know how to specify everything that affects how something sounds so we are therefore left with one and only valid way to determine how something sounds... listen to it. Before you even go there, yes, listening tests have many problems, but that's all we have as a final test.