Where does fatique come from?

Our ears use certain odd-ordered harmonics in order to determine loudness. We are very sensitive to these harmonics; if they are altered even by trace amounts we will develop 'listener fatigue' as it was know as in the old days.

Anything that can enhance these harmonics is a culprit: global loop feedback is a major sinner (which is why there are zero feedback amplifiers out there), non-linear devices such as transistors, transformers, cheap coupling capacitors, poor connections, breakup modes in loudspeaker drivers...

IMO a good system will not exhibit loudness artifact- no matter what volume level it is playing at, it will always be relaxed.

Fatigue might come from TIM distortions. In SS amplifiers with deep negative feedback output transistors go momentarily into saturation when feedback is too slow to respond to fast input signals (charge is trapped at semiconductor junction). These momentary gaps are not audible because our brains compensates by filling them. (causing fatigue).

You can find more about fatigue and TIM distortions in first paragraph here: http://www.valutronic.se/technique.html