I'm not sure I buy the ear fatigue argument.
I experienced a lot of ear fatigue when using my tube amp with a Suhr Reactive Load and a Torpedo CAB (for IRs). Also got a lot of (to a lesser degree) ear fatigue with my old Axe FX II. I can't say I've had much, if any, ear fatigue from the Kemper. It's safe to say that, from my experience, even tube amps can cause ear fatigue, it's not limited to digital modelling.
I'm sure that there are reasons I've not had it with the Kemper - lack of time spent playing it. Had it only a few weeks. Not spent a long time on loud. Not spent a long time with headphones. Many reasons, but highly doubt that it "being more natural" have much to do with it.
Sorry, hope that doesn't come across too negatively. Just trying to offer up a reasonable argument that tube amps can definitely cause ear fatigue.