Is it possible the "fizz" may be partially due to the low volume with high gain? I am no technical guru but as one who plays at home volumes a lot that fizzy thing is common (when low with gain) on several amps I have had. Until they reach that point where they open up enough to hear the amps voice. I call it the break over point and every amp I have had has one.
In other words, in my experience you don't get the full plate of tone until the amp is allowed to speak loud enough to get it all out. The fizz in my experience is like an amp that is choked and only a smidge of what it is trying to say is allowed to come out.
Now, hows that for technical prowess?
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I totaly agree !
It is also what I pulled of my amp's experiences
Every tube amp need some volume to sound good, under a certain volume, when you put some gain (from the amp or from pedal) the amp will sound "fuzzy" and "narrow".