Another very important point is that any SET is only producing 10% 2nd-order HD at its max output or very close to it. For this reason, I do think it's a good idea to plan a system with the SETs not required to go about 50% or so output. At that level, you're normally talking about a few percent THD, 90% 2nd harmonic, proven in testing to be entirely inaudible.
One study showed 7th order HD to be around 20x more objectionable than 2nd order.
One study showed 7th order HD to be around 20x more objectionable than 2nd order.