this is really a ying and yang type of argument.
one is of no value without the other

The state of the art of signal analysis is advanced to the point where
I do not believe it is possible for a difference in amplifiers to be
audible but not measurable. It may not be obvious at first what
measurement is needed, but once that is unraveled, some measurement
should always be able to explain any audible difference (talking about
amplifiers here, mics and loudspeakers are another matter)

If a critical listening test should uncover an audible difference
between two amps, the next step should be to make whatever measurements
are needed to identify the cause of the difference. Then the designers
can use that information to make design changes. Something like, amp A
has a more solid sound compared to amp B and we discovered that the
reason is the difference in damping factor (a trivial example).

Mark