THD has been published for amps since I was in college 50 years ago. Probably before then but I wasn't looking. It means nothing. It was always published because if a manufacturer didn't publish it, then the "educated buyer" would think their amps were inferior. Perhaps it was an indicator of quality of build, but I'm sure there was no agency checking for honesty in their ads.
You hit the nail on the head with your description of the Masterworks, "sounds much better; more clear, very lifelike". that is what matters. Maybe someday someone will figure out what to measure to improve on those qualities but it isn't THD. But my Pioneer was .05% thd. LOL.
BTW, I'm a physicist. I believe in science. I know what to meansure for many many qualities, but not for good sound. But I know it when I hear it and I know people who can design amps that produce it.
Jerry