This is a simulation, however these simulations fit very well with real world. Units are as in plots, Volts. 10V/1V is 20dB, as everyone knows.
Bob Cordell has been a big supporter of THD 30kHz measurements to reveal HF non-linearities and other hidden issues. And sorry, though Bob Cordell is an authority in amplifier design for me, @March Audio is not. Similarly Richard Cabot (AP) suggests more distortion test methods including HF THD and 19+20kHz IMD.
Again you are wandering off into the theoretical non real world.
The point is these IMD problems need to show up in band. A 19+20 test is in band not ultrasonic and something I repeatedly advocate. Its products at 38/40 are no where near as important as the direct 1, 18 or 20kHz. A low level of product at 38/40 will not generate further signicant in band imd issues, unless the distortion/non linearity is gross. but if there we can see the easily on an fft.
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