It has been my contention for quite some time that those with thorough engineering and science backgrounds are open minded and do not dismiss what listeners say they hear. Of course engineers and scientists use and rely heavily upon measurements. Yet they willingly concede measurements can’t (Yet) account for all that humans detect while listening.
@charles1dad back in the 1980s the bit in your last sentence was true. But since the newer analyzer tools have become available, no longer. I’ve noticed though that such has little impact since people doing measurements often don’t do enough or the right ones; manufacturers don’t always publish them and finally, there’s not enough education in the field to be able to interpret what the measurements imply.
But if the stars align then you can accurately predict how a circuit will sound, based entirely on the measurements.
Even in the case of capacitor coupling, loading the driver tube with a choke instead of a resistor will sound better - more dynamic and immediate sounding. No escape from good irons!
SET amps are simple, yet there are many circuit design and part choices that affect sound.
To the first sentence, I’ve not found that to be the case, although with any kind of inductive coupling good iron is essential! Choke coupling is mostly an excuse to use an expensive inductor when a good resistor will do as well with wider bandwidth. Put another way, if more time was spent optimizing the operating point of the tube and less on fancy inductors that wind up (if you see what I did there) being mostly for show and tell, the circuit would exhibit the same positive attributes.
Any circuit that is zero feedback will be affected by nearly everything in it since there is nothing to reject distortion, phase shift and the like. To this end, to avoid phase shift you need really wide bandwidth. To avoid distortion, you need good quality parts, each part of the circuit optimized for the operating point of the active device (since this issue applies to solid state circuits with zero feedback as well) and proper layout to avoid stability problems and noise pickup.