Sure, let's see a 18+19 kHz IMD and we will know. Especially at about -40 dB where actual music content is. Until then it is speculation.Nonlienearity at frequencies up to 20 kHz create difference intermodulation tones in audio band that are situated at subtraction of two frequencies or more complex in case of more frequencies. You cannot oversimplify the way you did. This amp’s high frequency linearity is poor. It is not true that distortion measurements have to be made only with 20 kHz BW. Lot of poor understanding may be seen in these non professional discussions.