Well said, Jeff (Jwm). I would add that besides the reaction of your friend being immature, it also reflects a lack of understanding of the fact that the sonic performance of the specific digital cable in the system in which he heard it may have been completely different than its performance in your system, due to differing interactions between its own technical characteristics and the technical characteristics of the components it was connecting. Even for a given cable type and a given system, differences in the length of a digital cable can result in completely different sonic effects, for reasons that are easily explainable. Those interested can read
this paper by A'gon member Audioengr (Steve Nugent of Empirical Audio).
Undoubtedly in many arguments about the sonic performance of audio components and cables those on both sides of the argument are correct, in the sense that they are accurately reporting what they heard. But what is often lacking on both sides is an understanding of the interactions and system dependencies that are involved. Interactions between amplifier output impedance and speaker impedance vs. frequency characteristics, for example, amplifier output impedance of course being very different between most tube amps and most solid state amps.
Best regards,
-- Al