Please let me contribute to not so often discussed theme of speaker distortion. I have done some research measurements on this topic in past years and would like to present it here. The simplest way is to put some links to my web page, so in case you are interested in the subject, please have a look.
Results of distortion measurements of a tweeter under voltage and current drive
http://pmacura.cz/speaker_dist3.htm
Introduction to speaker current non-linearity even in case it is driven from a perfect, distortion-less voltage source
http://pmacura.cz/speaker_dist.htm
Measurement of distortion of speaker current under voltage drive due to non-linear speaker impedance
http://pmacura.cz/speaker_dist1.htm
Results of distortion measurements of a tweeter under voltage and current drive
http://pmacura.cz/speaker_dist3.htm
Introduction to speaker current non-linearity even in case it is driven from a perfect, distortion-less voltage source
http://pmacura.cz/speaker_dist.htm
Measurement of distortion of speaker current under voltage drive due to non-linear speaker impedance
http://pmacura.cz/speaker_dist1.htm