It's just a tendency to distort the signal along odd or even harmonics. An odd harmonic is akin to playing things "in key" and the even harmonics is "out of key" so that music played on amps will have the signal distort ever so slightly along those harmonics. So a tube amp, even though much detail is not reproduced, because it causes the sound to be more "on key" sounds a little better than an integrated amp that makes it ever so slightly out of key (even harmonic).

Calvin