So how much can you tweak the tone stack on a direct profile before you lose the authentic sound of the real amp profiled?
Don't worry about authenticity of the sound. As paults said, if it sounds good – it is good. In the end of the day the only thing you are looking for is the sound, and it doesn't matter what name was written on the box. Kemper gives you ability to capture the sound you like and adjust it to your needs and taste. So look at tone stack (and other knobs) not as "going further from authenticity", but as "going towards to what I like and what I need". With all that, don't be afraid to adjust profile settings, they are there to help you balance profile sound with guitar/cab/frfr/etc.