it happens on all of my presets, regardless of pickups and cab selection. i normally don't use a mic either. i tried all of the amp settings including "ideal" and none clear it up.
the worse offender is the "singtall plexi crunch" preset. any cranked master volume makes the problem worse.
i have mic'd up a number of tube amps and never heard that kind of fizz. i just tried the same guitars on two different tube heads and the ghost fizz just isn't there at all. i tried the same guitars on a digitech rp1000 and a eleven rack and i don't hear it there either.
i hope its a simple setting to fix it.
some of my real amps (I have about a dozen tube amps) do it more than others, granted none as bad you clip.
Turn up the power tube bias all the way up, it should reduce it significantly.