Read the article about Nelson Pass and his harmonic distortion generator gizmo he gave away to some DIY people...https://positive-feedback.com/audio-discourse/the-pass-h2-harmonic-generator/
Excellent article in Stereophile
This is one of the most interesting articles I've read on harmonic distortion and its affect on sound quality and why the classic THD is a worthless specification.
https://www.stereophile.com/content/harmonic-convergence-effect-component-tweaking
https://www.stereophile.com/content/harmonic-convergence-effect-component-tweaking