Amplifier technology reached maturity in the seventies, when output transformers were abandoned in solid state amps. Ever since, well designed amplifiers used within their specifications have exceeded human hearing acuity.This statement is false.
Humans can easily hear the distortion of most modern transistor amplifiers. The brightness and hardness of solid state is well-known and a topic of endless debate- the brightness and hardness is the direct result of the fact that the human ear/brain system uses higher ordered harmonics to sense sound pressure. The slight amount of distortion is converted by the ear to a tonality. The ear has to be keenly sensitive to these harmonics because of the wide range of human hearing (140 db). So its far more sensitive to them than the lower ordered harmonics.
If anything has changed since the 1970s, its our understanding of human physiology. The audio industry has been a bit slow to respond to that knowledge though, probably because the facts are a bit of an inconvenient truth.