I forgot to say that THD figures are not very important by themselves nowadays.
Amplifier manufacturers choose a particular THD distortion number or level, and then read the power the amp puts out at that chosen distortion level.
this means that the THD figures are not a specification by themselves.

the very best amplifiers costing many thousands of $ will have distortion figures much higher than what appears in the specs when the amp is driven harder.

See: http://www.blu-ray.com/community/gallery.php?member=Gelli
or: Axiom Gallery