1. Are you using RME's Total Mix with the Fireface 800?
2. Are you seeing proper levels on your input channels in Total Mix?
3. Are you able to monitor the Kemper through the Fireface 800? Plug headphones or your monitors into the Fireface and set the levels with Total Mix (in Total Mix click on the output channel then raise the levels of the input channels).
4. Are you selecting both Fireface 800 input channels in your DAW (for stereo), or only 1 (giving you mono)?
If you use S/PDIF you will still want to monitor the levels in Total Mix.
I have used the Kemper with a Fireface UFX using the rear inputs and it works great, just make sure you have the Fireface inputs configured correctly in RME's Total Mix (there is a setup button on each channel that looks like a wrench).