Those two particular amps are "cathode biased Class-A" amps. IMO the designs are poor because... wait for it... they exhibit a papery buzz. The problem with most cathode biased Class A amps is, 1: they aren't really Class A (more like hot Class AB) and 2: They go into Class B operation when they are overdriven. The cathode capacitor charges up and the bias point shifts dramatically. This causes lots of crossover distortion (papery buzz). It's worse on lower notes because lower notes have more energy and charge the cap more.
The is probably the reason Leo Fender switched to fixed bias amps with negative feedback. Fixed bias is just that, the bias point is fixed so the amp doesn't shift into Class B operation when overdriven (not as much anyways, depends on the bias circuit, grid stoppers, etc.). Negative feedback linearizes the amp and reduces the crossover distortion (at the expense of gain). That's why the buzzing went away when you increased negative feedback. Crossover distortion is a unique sound. For cleaner sounds it tends to be objectionable. For overdriven sounds some find it desirable. EVH ostensibly liked his amps biased cold to get some crossover distortion. There are even some amps that have circuits to intentionally generate crossover distortion. Some distortion pedals also do this.
Lowering the Cathode Resistance reduces the crossover distortion as it keeps the amp in Class A operation longer but the tubes run hotter and don't last as long. This is not a problem with our virtual amps though. I forget the actual values but I'm pretty certain if you look at the Cathode Resistance value for those amps it's pretty high. This means the amps are biased somewhat cold to begin with and shift to very cold as soon as overdriven. One reason I like the AC-20 is that it runs the tubes hotter and exhibits less crossover distortion. The worst amp in this regard is the Badger 18. The cathode resistors are very large and the amp shifts into Class B operation early causing a spitty, buzzy distortion quality. This amp, however, is liked by many so that just goes to show that some people like that. It's like fuzz pedals. Some people like that spitty sound. I personally don't like it.
Then there's shared vs. split cathode. If you look at the schematics on the web the Maz-38 is shared cathode. I could never get the model to sound exactly like our reference amp. Finally I traced the entire circuit and our particular amp is a split cathode. As soon as I changed the model to split cathode it was spot on. This is not exposed to the user though (there's a hidden shared/split switch). Our particular amp says "Humbucker" inside the chassis. Not sure if the difference in the cathode circuit is because Dr. Z likes split cathode for humbucking guitars or what.
I could easily make the models more ideal and less buzzy but that's not my call. Accuracy always comes first. Most people want that authenticity so that's why the models are like that. If you don't like it either dial it out by reducing Cathode Resistance, increasing negative feedback, etc. or pick a different model.
tl;dr version. Those amps do that. Pick a different model.
