I believe that Canary Audio makes both zero NFB SET and push-pull 300b amplifiers. I don’t know if anyone has done a direct comparison between these two versions.
@charles1dad I have to imagine Canary has... PP amps, just like SETs, vary quite a lot depending on design concerns! For example, single-ended circuits exhibit in mathematical terms a quadratic non-linearity, which describes how the harmonic distortion arises. When measured, it is found that the math accurately predicts the harmonic content.
OTOH, a fully balanced differential circuit exhibits a cubic non-linearity. In this case the even orders are cancelled; ideally only odd orders remain, but they fall off at a much faster rate as the order of the harmonic is increased (with the 3rd harmonic masking them). This again is accurately predicted by the math.
So this is where I'm going with this: when you combine both single-ended and PP circuits in the same amp (such as seen in almost any traditional PP amp, like a Dyna ST70), you get both non-linearities. This tends to exacerbate the 5th harmonic (Norman Crowhurst also wrote about this problem).
So if the comparison is between an SET and and a traditional PP amp, you are getting more 5th than you would if the PP amp was fully balanced. I've no doubt this is a lot of why SETs can be preferred.
These are all readily solved engineering issues though- its not hard to build a PP amp that does not have that problem! This is part of what I meant when I used the phrase 'cast a wider net'.