Here's the thing about THD distortion. It mostly doesn't matter. Even relatively high levels of THD are not recognized by the ear.
The impacts of what amp designers do to lower it are far more noticeable than the added harmonics themselves.
And loudspeakers make orders of magnitudes more THD than amplifiers.
Factoid: the next time you see higher levels of THD listed on an equipment spec be attracted rather than repulsed. Higher levels of measured THD often signal a better sounding design than the opposite. (Not because they are higher, but because the design is likely more musical).
Measurements that do matter are mostly not mentioned: TIM (Transient Intermodulation Distortion), slew rate, or open loop stability are examples.
One measurement often included, but not understood, does matter. IM. Intermodulation distortion is quite noticeable to the ear and is often the red-haired stepchild, mentioned but not angst over.
Tomorrow we'll angst together.