I would say it's not so much about theory vs practice, but rather about the practical application
of theory
. Along these lines, I agree strongly with
@KenTajalli that there has to be a measurement standard, not only because many amps are being tested but also more deeply because the entire endeavor of relying on measurements is in turn based on using
fidelity as a standard. We use fidelity as a standard for the same reason that Ken cites for using 1kHz tones as a standard: not everyone has to prefer maximum neutrality/fidelity, but it's the only sensible standard since different people have different preferences for how they want their gear to be colored/"voiced." In fact, I'm always struck by self-identified subjectivists' hostility to measurements: if you like your gear to impart a "warm," "organic," "ambient," "tube-like" or whatever kind of sound, it's only by searching for deviations from maximum fidelity, through measurements, that you can make intelligent purchasing decisions. Otherwise you're just shooting in the dark and making almost random guesses about whether an amp you buy will sound "musical" or "sterile" - and this constant carousel of random purchases, along with a health dose of confirmation bias, is in fact what many audiophiles do and what keeps segments of the industry afloat.
I mention all this because, per your last comment, of course there's nothing wrong with measuring an amp at 15kHz if you are building one and you want to see if changes to the circuit produce changes in that measurement even if those changes do not produce changes when you measure at 1kHz.
But unless or until someone provides evidence of some repeatable mechanism or variable that can produce audible differences in an amp's sound and can only be measured by using higher-frequency tones, testing with a single 15kHz tone is not necessarily or helpful for someone who's reviewing or measuring multiple, already finalized and shipping amps made by multiple people/companies.
It's just a different use case for the use of the measurements.