Modeling amps such as the Kemper prefer a full range, flat response speaker, or "FRFR" for best results. Many of these FRFR speakers are incredibly expensive and appear to be little more than studio monitors repackaged as guitar-specific FRFR speakers. I'm sure that offends some people (Hey, I bought a Friedman ASR-12 as well) but hear me out.
I believe the frequency range of a guitar is primarily 70Hz-6kHz
Speakers in Hifi, Studio and Home Theater industries strive for 20Hz-20Khz which is generally considered the range of human hearing, though some can hear above 20kHz and you can feel bass below 20kHz. Most of the time dedicated subwoofers are used below 60-80Hz and the other speakers in a system will cover everything above that.
Any common, decent speaker for home theater, hifi or studio monitor applications will cover the entire "guitar range" of 70Hz-6kHz with ease and usually with a pretty flat frequency response if it's a larger speaker.
So...
It seems any decent speaker with a respectable frequency response would be adequate for use for guitar as an FRFR speaker. Many Home Theater center channels are horizontal and would be a great pedestal for a Kemper toaster to sit on top of. Most local classifieds are littered with great speakers like this at minimal cost. Also, very good "single" speakers can be found at a deep discount because no one wants to buy 1 speaker.
Another option, which I've seen discussed elsewhere is a single or pair of studio monitors, which have flat response as their primary goal.
While budget FRFR cabs make sense, I don't see any reason why you'd shell out for a high dollar FRFR cab when you could get a great studio monitor or other option above for a fraction of the price and be technically FRFR within the range you care about for guitar. It's also worth noting that some people prefer a regular guitar cabinet on their kemper, so a perfectly flat response is not always necessary for great results.