We need to consider that test gear has gotten better!!
When I was in school the distortion meter there was only good/accurate to 1% THD, today's meters can measure .001% THD. How would those old amps measure up on today test gear?
Tube amps distortion % increases as the power increases, SS amps the % of distortion decreases as the power goes up. All of this assumes that we are not going into clipping.
The plate transfer curve of a tube is NOT linear by any means. The most linear mode for a tube is a pentode in the ultra linear mode, push-pull.
A triode in push pull mode is next, single ended class A is good, BUT the power output is very low, because of the small linear part of the plate transfer curve.
With SS amps it is very easy to get .01% THD.