Nice clear and straightforward explanation, Bren.
Like you said, the characteristic distortion for solid-state amps is essentially revolves around the addition of odd-order harmonics, and such only occurs when the amp is overdriven.
Tube amps are 'easy to overdrive' but they distort gradually, and, actually, add in both even and odd-order harmonics to a sound (not only even-order harmonics).
I think that's really interesting. Here
is an interesting article on the subject of tube amps, though it mainly focuses on instrument-amplification. I have seen a mixer that had a build-in tube distortion circuit that could add-in a 'tube sound' to a recording during the mastering process. Very cool.
And hey, I'd love to hear a tube amp sometime instead of just hearing recordings of tube amp outputs if the offer's still open, 2x6spds.