I've found dozens of posts from people experiencing the same issues but I haven't found a fix. I have an Apollo Twin and an RME Fireface 800 and the Kemper sounds significantly worse in both compared to headphones. It almost sounds like a dry guitar signal with effects layered on top of it, or if the levels of guitar signal/amp/cab/effects are screwed up with the volume of the amp and cab turned way down. Here's what I've tried so far.
Tried TRS, TS, and XLR cables. No audible difference.
Turned off "Space" in the headphone settings
Tried the guitar input on my Apollo
Tried every option under Main Output settings (Mod Mono, Mod Stereo, Main Mono, etc.)
I'm at a loss and hoping someone has some ideas.