The output level is the same whether I use a cab sim or not. Would be very unpractical to have to change the output level. The tone has also too much treble when I go out on the GPA100 without cab sim. Actually, Snaptone + Cab IR in the GPA100/PPC212 sounds more like the real stuff than Snaptone without Cab IR in the GPA100, which makes no sense. I'll try reducing output level when the output is the GPA100.Check your output levels.
When I had the Ampero 2 Stomp, I did not find that turning cabs off had this sort of effect. Because cab sim -> guitar cab sounds like you have a blanket on, maybe this is what you perceive as "aggressive"? Any clipping would likely be caused by too high levels somewhere in the signal chain.