Harmonic Distortion In Maggies

Sure they can be rated.

Get a soundcard, measurement mic and you can test 'em urself!

Nothing to difficult to do since the distortion specs are likely >0.1% best case at standard listening levels (for the most part) and rising with level, and as one goes lower in freq for a given driver (usually).

A two tone CCIR type IM test is even more revealing.

The THD is likely published in some magazine's reviews... fyi.