In my quest to make better amp buying decisions. I have been reading about amp measurements lately.  From what I understand, one of the measurements called Total Harmonic Distortion(THD) measures the difference between the sound wave coming into the Amp and the sound coming out.  The difference is a THD percentage. The lower the THD the more accurate the amp is in amplifying the signal.

Question, are these measurements reliable and does it reflect real world performance?  Does low THD measurements( < 1%) always mean good amp?

There have been studies done that suggest human's can't detect harmonic distortion until it exceeds 3%.

If you were to walk in to a stereo store and blindly pick the amplifier with the highest THD odds would be heavily in your favor that it would be the best sounding amplifier in the store. The reason is because ultra low THD specs are the result of high amounts of negative feedback and it has been established that amps with less or no negative feedback sound better.

Three things that are helpful to know about THD.  1) There is odd order harmonic distortion commonly found in push pull amplifiers and almost all solid state is push pull.  2) There is even order harmonic distortion commonly found in single ended tube gear.  3) You experience neither one of these forms of distortion at normal listening levels so in reality THD has no real correlation on how an amplifier will sound.


Side note:

Your research is going to lead to the realization that there are two camps that audio products can fall into. Camp A) Products created by "math guys who design everything on paper to have the max power and lowest distortion" and "amplifier designers who actually know what their doing". The first is the overwhelming majority that create and sell most of today's audio gear... which explains why so much audio gear suffers from mediocrity. The fact that so much of it does sound the same has created a marketing dilemma which has spawned a concept called "audio jewelry" where the bulk of the money spent in manufacturing is now on appearance, fit and finish, the audio quality being secondary even though it is advertised to be uniquely superior.