I simply placed a Piezoelectric Compression Driver Coupled in a Dispersion Horn in series on top of my openback cab with two Celestion g12t-75's. I took it out of a Squier SQ12M.
The Kemper cab profiles remain ON, like for FRFR speakers. The combination sounds great! IMHO much better than any cab alone, and it has more room mojo than my good monitors. I'm gonna box this and market it as "Cab Saver"!