Tam, check the amplifier lab measurements, such as those at SoundStage which I mentioned. All amps produce both even and odd order distortion, in varying proportions. The use of tubes can result in 2nd order distortion being higher than third, or the third being higher, depending on the overall design(e.g. using a balanced push-pull configuration to partially cancel the even order components).

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Enjoy the music, not the equipment.