Not only that, but what if his so-called improvement makes him THINK
it sounds better? Isn't that in many cases the same thing as
sounding better? After all, the human auditory system was never
designed to be simply a measuring device.

Why not just say ommmmm, until we think our audio systems sound better?

Those of us that "know better" probably won't be motivated by the power of
suggestion. Take a look at the recent (ie. recurring) amplifier threads in
RAC to see that people do indeed think they hear a difference when a
difference demonstrably does not exist (or if the difference is a product of
something else). So, for these people, is there truly a difference between
hearing a difference by power of suggestion rather than by actual acoustical
differences?