For those of us that don't quite understand the difference between odd and even order distortion and the difference it makes I have included a piece of an article that will help explain some of it.

As you probably know, harmonics (also called overtones) are what make up the timbre of
each instrument so that the A440 of a clarinet sounds different from the A440 of a
saxophone. The sax tone is rich in harmonics while the clarinet sound is almost pure
fundamental without overtones. Certain harmonics occur naturally in music -- both in the
overtones of timbre and as component notes in the harmony of chords. These overtones are
often called the "low order" or "even order" harmonics such as the 2nd, 3rd and 4th. These
overtones relate to the fundamental note by being one octave above (double the frequency);
an octave and a fifth above (called the "quint" in pipe organs, an octave and a half above)
and two octaves above for the 4th.

The ear has a great tolerance for these overtones - because they relate in a musically
consonant way and are are already present in the original signal, as either timbre overtones,
upper harmony or both. For example, it is often possible to add up to 20% second harmonic
to a single note before any change is audible. Then all that happens is that the ear hears the
timbre change.. it does not hear "distortion trash." If you're a guitar player or keyboardist try this simple experiment. Play two notes, play the low one loudly and the high one very soft at
first, then increasingly loud, an octave apart at the same time. Notice how loudly you can
play the upper octave note before you can even hear it... and that what you do eventually
hear sounds mainly like a timbre change in the lower note -- not two separate notes. Now try
playing the same original note but this time also play a note two octaves plus one step
above. No matter how softly you play this note, it is distinctly audible and separate from the
original.

What we've done with this experiment is demonstrate the effects of different harmonics in
contrasting distortion types. You've heard how inoffensive the low order (sometimes called
"even order") harmonics can be... especially compared to the prominence of the upper order
(often called "odd order"). And as you might guess by now, tube amp distortion is primarily
low order while that of most transistor units is odd-higher order... and hence very
objectionable. Clearly then, a tube power amp with a higher percentage of Total Harmonic
Distortion actually can sound cleaner and clearer and much less harsh than a solid state
amp which carries a lower total distortion rating. It's not the total distortion that counts as much as the distribution of distortion components: It's all but impossible to hear even a few percent of 2nd harmonic distortion whereas a fraction of "high order" type becomes audible
as annoying hash.