So guys, I tried a lot and got the most comfortable wirth the connection via usb from Kemper to pc. Thank you for all your help and ideas!
Finally I tried with a normal guitar cable, had the same issue. The volume level was pretty decent.
musicmad I had no plugins or even effects active in cubase. I recorded stereo via two input channels (left and right), the sound didnt change when I muted one channel exept the volume. There was no other technology than cubase active. Live playing and playback had the same volume, headphones also made no difference.
Michael_dk I use the KRK V6 as monitors, while recording and playback, I tried different setups with my headphones (akg k141) but the result was the same.
Bakersounds That video helped me pretty much for the beginning, I searched for some other videos and now the usb is (at the moment) my preferred method.
Maybe that´ll change in the future but we´ll see.
Thanks again everyone!