Negative feedback cannot strip harmonics and this can be easily measured. If anything, negative feedback widens bandwidth and lowers distortion. Improperly used can lead to Transient Intermodulation distortions (TIM), that add brightness to the sound (higher order odd harmonics).
On the other hand more distortions without feedback can make amp sound more vivid, pretty much the same way as distorted guitar (a lot of harmonics) sounds more dynamic than clean Jazz guitar at the same loudness. Sound that is "warmer" than neutral has even harmonics added by an amp. Adding even harmonics might sound nice but reduces clarity.
On the other hand more distortions without feedback can make amp sound more vivid, pretty much the same way as distorted guitar (a lot of harmonics) sounds more dynamic than clean Jazz guitar at the same loudness. Sound that is "warmer" than neutral has even harmonics added by an amp. Adding even harmonics might sound nice but reduces clarity.