What determines good distortion?

What I've always heard is that SS produces odd order harmonic distortions (3rd,5th, etc) while tubes produce even order distortions (2nd, 4th, etc). I've also heard that musical instruments produce even order harmonic distortions, which is why most folks find tube equipment more "musical" or "natural", because it's distortions are more similar to that heard from a musical instrument.

J, I think you are mistaking tube for single ended and ss for ultralinear. An old Pass Aleph does not distroy even order harmonics like an old ARC push pull.

You will hear a lot about ss sound and distortions vs tube sound and distortions. First, eliminate the stupid. A well designed and built amp will be wonderful, regardless of whether it is tube or ss. As Jmcgrogan2 stated, ss designs "typically" produce odd order harmonic distortions, where tubes produce even order distortions. This is only true so far as how the amps were designed. circuit designs can correct for distortion effects. it totally depends on how much effort and money the designers placed into their designs. So, please do not buy into the ss sound one way and tubes sound another way. you can have a tube design that is basically crappy circuit design, same as with ss. So, it is a combination of the actual circuit designs (regardless of whether it is tube or ss) and then you add in whether they "corrected" for particular distortion effects in the design. I have heard (within specific price points) ss amps that were much better than tube amps and visa versa (within the price points). At the upper end of the price range, i.e. rediculously priced amps, I would take either one. The ss amps sounded wonderful and so did the tube amps. One is definitely not better than the other. It depends on price, circuit designs and other factors. Please don't buy into the tube is better than solid state or visa versa arguments. Pick a price point, then go demo in your home, using your equipment the various amps and see/hear for yourself. depending on the amps, you will probably pick a ss amp and in other cases, the tube amp will make you smile.

stay away from the hype.

enjoy

Jmcgrogan2 is correct about odd and even order harmonic distortions. The best explanation I've read to explain the difference is this:
imagine you are listenning a sollo singer - this is your base sound. Now imagine you are listenning to a chorus - thouse are all your harmonic distortions. If every singer in the chorus sings in cynch (even order harmonic distortions) the sound, even though is different from a solo singer, is pleasant. However, if anybody in the chorus is singing out of cynch (odd order harmonic distortions) the sound becomes unpleasant.

Lewhite is correct, I was confused. It's single-ended amps that produce the even order harmonic distortion versus push-pull amps which produce odd order harmonic distortion. I think my confusion is because MOST single ended amps are of the tube variety. However, yes, the Pass Aleph series are single ended solid state amps.