I wondered if it was something like that. My expectation was that these virtual pedals would work just like the real thing. I know what to expect when I plug my guitar into a real Fuzz Face. I was further thrown by the Tube Screamer which seemed to distort as I expected.
I suppose, thinking about it, there is no way for the simulated Fuzz Face to include the guitars passive electronics in its input circuit and interact with it, as the real Fuzz Face does.
My patch was very simple - just the Fuzz in line.
I'll experiment with the levels.