Distortion Distortion Distortion

 I’m 73. I have a high end system. I became very sensitive to distortion when in my 30's... not the measured kind... the stuff that comes out of speakers from electronic components. I have never really been able to correlate measured distortion with what I hear. Often components with "measured low levels of distortion"... sound terrible. The lack of correlation with measurements has been true with most stuff in audio... except the most elementary. 

While I am sure my hearing isn’t as good as it used to be, I instantly can detect distortion. Often it is more a pressure on the ears... as opposed to something you actually can focus your mind’s eye on hearing. But my sensitivity has not gone away. And, the lower the distortion, the lower the volume I can listen at and enjoy the dynamics. The volume I listen at continues to go down. 

@bjcatanzaro  Mine is DAC3 + AHB2.  I agree - it is as clean as it gets.  The only negative part,  according to many, is the same thing I praise it for - lack of distortions.  Many amps add euphonic second and third harmonics  covering unpleasant higher order odd harmonics in the system.  It often helps with bright records or hot tweeters.  Some people also find Benchmark gear sterile sounding.  Either added distortions make sound more dynamic (like distorted guitar vs clean Jazz guitar) or we got used to sound with limited resolution.  I remember post when somebody complained that instruments don’t sound together (sound blob?).  As Benchmark’s technical director John Siau stated that there is nothing "wrong" with any sound.  It is just a matter of taste.  He is in "purists camp" claiming, that some instruments won’t even sound right in "warm" system.  Piano overtones, for instance don’t follow exact harmonics, because of stretched tuning.  Octave higher note is not exactly double frequency.  In overly warm system these overtones can "beat" with harmonics making piano sound almost like "out of tune".

There is also distortion of a circuit in static conditions, and distortion under dynamic conditions. Take feedback- if the circuit is large and it takes measurable time to pass the signal, feedback will no longer be accurate enough to reduce distortion, it will be additive (transient inter modulation distortion and slew-induced distortion).

Simple circuits often sound best.

Also, bipolar transistors are inherently exponential devices, leading to odd order harmonics that are easily heard. Tubes are square law devices, and typically produce even order harmonic distortions which are additive and so not so jarring (tube warmth). These are generalizations.

@bjcatanzaro Distortion, for the most part, is the differences you hear between components. For example the smoother tonality often attributed to tube equipment is due to harmonic distortion.

Our ears use higher ordered harmonics to sense sound pressure to are keenly attuned to them. This is why older solid state amps often sounded bright even though on the bench they measure perfectly flat.