I will experimenting with measuring harmonic distortion... I have wanted to do these graphs for the longest time!!!

Measurements were taken with headphones calibrated to 87db A-weighted SPL with white noise. D2 - D5 in the below graphs represent 2nd to 5th harmonics at each frequency. Short primer: ideally when a certain tone is played, say a 100Hz sine wave, the headphone should only reproduce the 100Hz tone, and nothing else. However, in the real world, when we play a 100Hz sine wave, we will see all sorts of garbage (distortion). Any extraneous tones being reproduced which are multiples of the original tone are called harmonic distortion. For a 100Hz tone, 2nd order distortion is 200Hz (100x2). For a 500Hz tone, 4th order distortion is 2000Hz (4x5).

Below is the HP1000 harmonic distortion graph. It measures fairly flat in the bass, but yet it sounds quite bassy. This is because of the high level of 2nd harmonic distortion. The second harmonic amounts to another bass tone being played (less loudly) exactly one octave higher to augment the original tone. I will post post other headphones for comparison. BTW heavy 3rd order harmonics in the bass tends to make it sound one-note, undefined, or indistinct. Lots of overall harmonic distortion makes bass sound muddy. There is no such thing as "good" distortion.

Finally, note that the lines below represent specific orders of harmonic distortion, not THD or total harmonic distortion. I think it's fair to say that 1% or even 0.5% is easily audible.