I think that you are in search for specific THD spectrum from power amplifiers and not for some 0.00000001% ultra low THD amp. , usually that `Single-End` THD spectrum consists from leading dominant second harmonic followed by lower level third harmonic , fourth, fifth and other higher harmonics need to be significantly suppressed up to -100dB or even more ,
amplifiers with opposite THD spectrum where leading dominant harmonic is third harmonic , followed by low level second harmonic is characteristic for quality Push-Pull amplifiers , here higher harmonics need to be significantly suppressed to , in both case those harmonics rise and fall in Monotonic way depending from input signal level ,
that sort of amplifiers is called or named `Euphonic amplifiers` , but not so much Hi-Fi , surprisingly usually those amps sound very good to human ears ,specially at long listening sessions , they do not burden human ears very much ,
here`s one example of such small simple `Euphonic` amp. which consists only from four transistor , THD % is not so good , almost mediocre , but THD spectrum is almost as is come from some Triode Single End tube amplifier .
Best Regards