I tested a lot of speakers and cabs for the Kemper (different Celestions, EVoice ect.) and I never was really satisfied with the sound. I have to say that I don't use FRFR-cabs anymore, for me a guitar-cab with Kemper's cab-sim off is the better choice.
Now I found a really good and not very expensive speaker: WGS ET-65
I even don't have to change settinges/eq and can use my profiles optimized for studio and tours (direct to desk). If you're looking for a good speaker, try it out.