I just tried using the kemper as the guitar CAB only. Here is my Jet City JCA20H head straight into THD Hotplate and then the line out from that into the alternative input on the back of the Kemper. I just picked a kemper rig at random and switched off the kemper amp so I'm just using the CAB.
I've been so busy messing about with the kemper that I haven't even bothered to change my strings since I got the thing and they do sound like very old strings as they don't have that "sizzle". Anyway, I think it sounds great using it this way. I profiled the JCA20H this morning too (just the amp without the cab). Although the profile sounded similar, in the feel department it was only about 50% there of the real tubes.