I'm quite new to the KPA but I can tell you how I do it.
I'm the oldschool guy who still prefers his fullstack in the rehearsal room. When playing live the changover times are short and there's often backline that is already miced up so we just plug into the cab(s) available.
My approach is to find a profile that has a cab which is similar to my cab. If you have a Marshall 4x12 with V30s or T75s you should be able to find or buy some decent profiles. I recommend merged profiles (packs) so you have the best sound possible via cabinet and main out. Tweak you profile so that it sound best via main out and than do necessary adjustments for your specific cab sound with the monitor EQ in the output section. This might also be necessary if you play over a different cabinet or if the cab is placed differently in the room. This way I can have my blasting cabinet sound (I often add some bass) and the FOH is happy because you made sure it sounds good via main out in the first place.
I play through a fullstack consisting of a Marshall JCM800 cab with T75s and a Marshall 1960 cab with greenbacks. Sounds awesome!
I'd recommend you get a used Marshall 4x12. You can usually get one pretty cheap and there's just nothing like a guitar cab working hard to shake your bones.