But, is 3rd order harmonic distortion as easily/naturally tolerated by the human ear as is 2nd order harmonic distortion which is quite well tolerated. Some would say that in fact 2nd order Harmonic distortion is congruent with human hearing.@charles1dad
The short answer is 'yes'. The ear treats both *exactly* the same. What *does not work* is if the amp has low distortion, but the distortion it has is mostly higher ordered harmonics (5th and above). Such amps tend to sound bright and harsh, compared to an amplifier that has a predominate 2nd or 3rd. The reason that so many push-pull amps get docked for their sound as opposed to an SET is that when you combine a single-ended voltage amplifier with a push-pull output, that type of topology tends to produce a noticeable 5th harmonic, which makes the amp less musical.
The only way I've found to get around that is to avoid that sort of topology- either go entirely single-ended and get that 2nd harmonic or go entirely fully differential and get a 3rd harmonic (although at about 1/10th the level that the 2nd shows up in a single-ended circuit). Since the ear is relatively insensitive to either other than contributing to 'warmth', 'body' and the like, this seems to be the way to go since you can't put enough feedback on a tube amp to really tame the distortion properly (or most solid state amps for that matter)- you wind up just making it brighter and harsher.