When an amplifier is driven with a signal, the signal at the output terminal of the amplifier is an amplified version of the input signal PLUS any distortion created by the amplifier. All amplifiers and signal processors add distortion to the signal. The levels of harmonic distortion in a high quality amplifiers are sooooo far below audibility that they are basically of no concern. When someone tells you that one amplifier is definitely going to sound better because its THD is rated at .002% vs an amp with .05%, you can be pretty sure that he doesn't know what he's talking about. Many people believe that THD below 1% is not audible and I believe they're probably correct. The fact is, especially in an automobile, the amplifier is the least of your worries. Speakers used for mids and highs commonly produce between 1% an 5% THD and for subwoofers the numbers can easily reach 10%. This doesn't even take into account all of the resonances of all of the plastic and metal panels in the vehicle.

This tone starts off 'clean'. At a point 5 seconds from the start, a 1% 2khz signal is mixed with it. At start +10 seconds, the 2khz level increases to 5% for a 5% total harmonic distortion. To hear the transition between 0 and 1% you may need to use headphones. You may have trouble hearing it through the computer's speakers. When the harmonics reach 5%, they are clearly audible. If the tone was music, you would have trouble hearing 5% THD. The 1 % is only barely audible and it's mixed with a continuous, steady tone. With music, it would be inaudible. This little exercise is just to show you how little 1% THD is and to help you realize that .000... % THD is useless as an indicator of sound quality.