I remember the private demo I was once given by Quad's late owner/designer Peter Walker. It was in the days when hifi journalism had started to criticise his amplifier designs, claiming that his successive designs had become increasingly 'unmusical', with his valve amplifier touted as the best, and his most modern 405 current dumping design as the worst. He had bravely commissioned an independent double blind test to respond to the criticism, that was subsequently published in the authorative journal Wireless World:
https://linearaudio.nl/sites/linearaudio.net/files/Valves versus Transistors DCD.pdf Needless to say, even very experienced professional listeners could not distinguish them in this blind test. It was my privilege to enjoy a private version of the test using a Studer recorder playing original master tapes, and it was educating. I had believed that I would have been able to tell them apart, and I thought I did succesfully, until Peter smilingly showed that I had not been better than random. He was a real gentleman, and on that occasion persuaded me that there was no point in upgrading my 303 amplifier to his 405 playing into my Quad ELS 57 speakers ('you would be wasting your money'). Of course, in those days DACs did not yet exist outside of laboratories.