Today I was setting my gear up for a church gig, and decided to use an excellent TAF profile of a '63 Vibroverb (a free sample IIRC). I am using a pair of Bose open ear earbuds. The Kemper is set with the PureCab on around 1.7, and the Space parameter was set to 5 (has v3.1.4 installed). In order to set up the profile correctly in Performance mode, I muted the Kemper in the mains and plugged the ear buds directly into the headphone output of the Kemper. I then hit a big chord..
... and had a huge fright as the Kemper's tone was filling the room, sounded like a loud amp reverberating the walls. I thought it wasn't muted in the mains after all; but indeed it was. I asked someone on stage if they could hear me, and they said they couldn't. So I pulled the ear buds out and hit the same chord, could hear nothing but the strings on the guitar. Put the ear buds back in, and the room seemed to be shaking again.
It was such a weird visceral sensation; apparently a combination of the reverb on the profile, along with the Space parameter and PureCab, made the tone sound and react through the in ear monitors exactly like there was a loud amp in the room. Even though I knew I wasn't making any sound, I still had to fight the urge to turn it down as it sounded like it would disturb someone in the room with the volume.
I've never had this reaction with a modeler before. When it gets so close in tonal results that you have a nearly uncontrollable urge to cut down the volume as it sounds like it is too loud in the room, but it's not even audible except through your ear buds, maybe you are on to something.