FWIW, low ordered distortion is pleasant, but it does obscure detail, and the ear tends to hear it as a fatness or warmth in the sound. Lower orders are the 2nd, 3rd and 4th.
Transistor amps have almost none of these distortions, but they are common with SET amplifiers. P-P tube amps on the other hand tend towards the 3rd and 5th.
Now it is the higher odd orders that our ears use as loudness cues. Anytime they are distorted, the amp will sound louder than it really is. This is why SETs will sound very 'dynamic' for their power, as normally they don't make much distortion, but if you push them (which is what transients do) then the loudness cues appear **but only on the transients**.
Transistor amps tend to have these higher odd orders all the time. This is one of the reasons they tend to sound hard or bright. Now its important to note that these harmonics do not have to be very distorted, usually 100th of a percent are audible, simply because these harmonics are so important to the human ear.
This, BTW, is why two amps can seem to have such different tonal characteristics even though they both measure flat frequency response on the bench. The addition of global negative feedback to any amplifier will increase the odd-ordered distortion slightly, which is why any amp with GNF will tend to sound brighter even though frequency response is unaltered.
(The trick, IMO, is to build an amplifier that does not use feedback, and use other means to eliminate distortion.)
The bottom line here is that distortion is always audible and to nearly anyone. Its just that it does not *sound* like distortion to us, often it sounds like a tonal aberration ('bright' in the case of many transistor amps, 'caramel' in the case of many tube amps).
