How Much Distortion Can We Hear With Music?

An interesting experimental study by Axiom reveals that we are unable to easy detect distortion at lower frequencies.
http://www.axiomaudio.com/distortion

what these results show is that our detection threshold for “noise” (made up of harmonically related and non-harmonically related test tones) is practically non-existent at low frequencies.

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In fact, the “noise” tones at 20 Hz and 40 Hz had to be increased to levels louder than the music itself before we even noticed them.

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Here is another piece talking about loudspeaker distortions.
http://www.enjoythemusic.com/magazine/bas/0708/

The loudspeaker is by far the poorest link in the reproduction chain for all forms of distortion, and this includes nonlinear distortion, frequency response distortion, phase distortion, and time distortion.

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I am fed up with individuals who claim they can hear the difference between a power amplifier with 0.05 and 0.01 percent distortion while ignoring distortion that is 10 to 100 times greater in the loudspeakers.

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Excellent articles.

I have tried to follow this advice about purchasing stereo systems:

Stores have a tendency to bundle medium quality equipment with cheap speakers. Therefore consider upgrading the speakers.

Upgrading speaker quality is the main factor that will bring an audible improvement at your ears.

Out of the total price you pay, put 50 percent toward the loudspeakers.

Every person has a different capability to hear clear tones and distortion. Many papers on physiology of hearing were written but nobody understands the problem.
This is why the technicians designed distortion meters that quantify the harmonics of a clean tone which define the distortion.
On the other hand, some people can perceive other distortion not related to the harmonics.
Generally, for frequencies below ~200-400 Hz, very few listeners can perceive distortion, and only at a high volume. Most listeners can perceive distortion for >400 Hz as harmonics. People with "absolute" hearing can perceive as a distortion also tones off the common scale, like in Arabic and Hindu scales.

Even if you buy the best available speakers, the effect of other objects, room reverberation and presence or absence of persons will affect your perception.This is why good headphones are often preferred as only instrumental distortion is present and can be technically reduced.