Its mainly hearsay and plus these two places:Where did you read this?
Odd vs Even Harmonic Distortion (What's The Difference?)
A simple, easy-to-understand guide on the principles of odd vs even harmonic distortion in music production. When to use each, the maths, and differences.
producerhive.com
Do even order harmonics sound better than odd order in amplifiers? Does it sound good?
Some peeps say that some amps have odd order harmonics therefore the sound will be OK versus having even order harmonics. Is this true? Note: If I worded the title poorly let me know and I'll adjust it so it more sensible.
www.audiosciencereview.com