I forgot to say that THD figures are not very important by themselves nowadays.
Amplifier manufacturers choose a particular THD distortion number or level, and then read the power the amp puts out at that chosen distortion level.
this means that the THD figures are not a specification by themselves.

the very best amplifiers costing many thousands of $ will have distortion figures much higher than what appears in the specs when the amp is driven harder.

All I know is you can drive these amps to dim the lights in the house & they are on their own circut w/200amp service. The M 80s are crystal clear as are QS 8s. Song over, dead silence.
I really like em.

To add though, when I run my 27K generator, lights don't dim. Hmmmm