An interesting aspect of THD when dealing with audio is that it doesn't tell the whole story unless the spectrum of the distortion components is known. While low even order harmonics of an audio signal might actually contribute to the sound (as they are the same tone in higher octaves, they will sound musical), high odd order harmonics will sound very bad (they bear no relation to the original tone from an audio perspective) thus decreasing the listening pleasure as the ear/brain perceives it as noise. This is why tube amplifiers still are popular amongst audio enthusiasts; they produce most of their distortion as low order harmonics contrary to modern silicon, so the sound produced is more "pleasing" to the ear even though tube gear have inferior THD specs when compared to modern devices. So in audio gear, you cannot judge quality by THD specs alone without knowing the spectrum.
