Hello!
I've finished my Monitor Project by putting an Beyma 12ga50 in a cheap harley-benton 1x12 cab (from thomann) , close the back (!!! makes more definition and pressure) and listen. It's an amazing monitor-cab for the Kemper, just tweak a little more bass (I've made an extra Output preset for it, so I can switch fast) and it sounds very good for this price (payed 105 € for the Beyma and 25€ for the used cab)!!! I compare it with the headphone Sound with a AKG 712 Pro and it comes very close! Maybe I have to tell that I damp the inner sides and the back of the cab, because it has two bass-reflex-holes (it comes with that holes, I leave it open) and I don't want to hear the inner reflections as much - I would recommend this! I think it's now a good Monitor cab for rehearsal and stage/live gigs.
Have the Beyma in my VOX nighttrain-cab with open back before, but I think the cab is too small for the beyma, too less bass. so I put the greenback back into the Vox and that is my second experience, original VOX nighttrain 1x12 open back cab, which is very little and there's an Celestion G12M Greenback in it. This cab sounds also very amazing with the Kemper (I leave the monitor cab on!!!), not that flat like the beyma, but much more amp-sounding color, which is very great. I use it in my homestudio for composing and practice, and it really rocks! Buy it used on eBay for 120.-€, it's solid and very Little for an 1x12 Cab.
I forgot, the beyma has now break in, makes a deeper bass and the fizzy heights before has gone. I think it's a good Speaker, when it's in the rigth cabinet - in my opinion it sounds better with a closed and a bit damped cab.
Greetings
Andreas