THD is a measure of non-linearity, which adds harmonics not present in the original signal. Yes, the standard tests use sine waves, but the results are applicab

THD is a measure of non-linearity, which adds harmonics not present in the original signal. Yes, the standard tests use sine waves, but the results are applicable to other signals. Just because most people don't listen to sine waves for fun doesn't mean it's a valid measurement. On the other hand, the highest THD in a sound reproduction system comes from the physical transducers. Worrying about the THD of an amplifier makes little sense when the speakers provide an order of magnitude more distortion.