Verdinut,
Yes, a figure at say 500 Hz or 1 KHz would be more meaningful, if that is all one is capable of measuring.
Distortion measurements are more meaningful if they could also include harmonic components separately instead of the sum of the total, because some products e.g 7th, 9th and 11th harmonics (yes, that high up!) can be so troublesome that even near the threshold of hearing they can cause 'stridency'. (Note that the culprits are mainly odd order harmonics. Even order ones are so many octaves higher, hardly disturbing in small quantities.) It refers a.o. to a phenomenon called listener's fatigue, about which you can read up om the internet.
Previous research created knowledge of how hearing reacts to such, which is then at present used to determine any possible problems. You can get very good info regarding this on the internet. The story can become involved, but experience helped to 'create' a model of hearing which one can then use to get pretty fair piture of how 'clean' an amplifier is. (Mercifully for us there are auditory limits of human hearing which is to our advantage.)