Hey Juan. What about the sub harmonic frequencies that occur with stiffer power supplies (like Marshall used) that result in "beating".?. Sum/difference harmonics that occur IN the audio spectrum BECAUSE there are frequencies below the audio spectrum. This is actually a cool effect sometimes and probably wouldn't happen if the amp weren't trying to reproduce frequencies below "guitar amp" spectrum. It's just another mojo part of the coveted tonal qualities of our vintage favorites. I'm sure there are similar phenomenon happening in the HF as well. May be more or less relevant here. Just keeping it real
EDIT: Then there's Enzo's consideration of context. At this level it's more about listening tests than it is about loop and pole theory (I think). Actually understanding "beating" probably won't even help a designer to reproduce it short of the original designs that do it already because there are, indeed, other complex influences in play. Trying to isolate them with even the most advanced bench gear (or believing you have?) hasn't proved out much in the real world. Nothing should be ignored, but anything claimed should be proven. I think that's where we are in this thread.