An excellent lecture held at UCSD adresses the difficulties of measuring distortion and relate it to perceived sound quality can be found here
http://www.performancerecordings.com/capturing-music.htmlThe reason for adressing this issue is that from an engineering standpoint I can not believe why most manufacturers fail to provide even the simplest of distortion parameters. Many so called high-end HiFi manufacturers go on and on about what space-age materials they use in their membranes and such but do not publish any numbers to back up their claims for superior sound quality. True - music and sound reproduction is best judged by listening rather than looking at tables or graphs. But if their driver is so damn good - it will show in the specs!
My 0.05
/Magnus