Had a Kemper using friend call me up the other day - he's enjoying his Kemper but having an issue:
When he finds a profile that sounds good with his 4x12 cab and Matrix power amp, he's not so happy with the FOH sound and when he gets a direct/FOH tone he likes, it doesn't sound good to him through the 4x12/Matrix.
My suggestion: Profile his amp with the line out (no cab) and with the monitor out from the Kemper going to his 4x12/Matrix setup. Dial it in exactly how he wants it. Profile it, then just start auditioning cabs for his FOH tone. get the 4x12/Matrix sounding right FIRST, then add the cab/speaker emulation to his FOH stuff.
Personally, I prefer using full range powered monitors onstage, but can understand if someone doesn't want to give up the power/coolness of a 4x12 cranked behind you. Not sure if this would work for everyone, but I think it might help!