Skylab
Reviewerus Prolificus
The very simplified answer is that when it comes to harmonic distortion, odd-order harmonics are generally viewed as more troubling when present than even-order harmonics. Because even-order harmonics are consonant with the music, they are much less objectionable. This is why a SS amp with a spec of .1% THD can sound far worse than a tube amp with 1%, if most of the SS amp's THD is composed of odd-order harmonics.