That is because most other digital "modeling" amps do not accurately or authentically duplicate the dynamics of a tube amp's power stage, and the impedance interactions between the power section and a connected guitar speaker. Most companies that offer solid-state power amplification, which are geared for guitar preamps, deliberately design their amplification to intentionally emulate the response of a tube power amp, and the "color" that they characteristically impart.
This is already taken care of by the KPA in the profiling process, which clones not only the reference amp's preamp but also it's poweramp, including the dynamic interactions with the connected load (speaker). Therefore, Kemper specifically recommends using a transparent and flat power amp for their non-powered units. This is also why Kemper selected the IcePower Class D poweramp (powered KPA units) for it's clean, flat and transparent amplification.
Cheers,
John
Right, but given that guitar amps and the speakers you are emulating aren't by any means flat, a lot of these companies seek to design something that delivers an accurate representation but also behaves like an amplifier. The Mission Engineering stuff in particular feeds back like a traditional amp despite being FRFR where it seems a lot of other FRFR solutions don't, especially PA speakers. It's a plus for me that it behaves like a guitar speaker should while not recoloring the sound.
When Bryan Turner reviewed the Mission Gemini for Guitar Magazine, he had a powered and unpowered Kemper. To compare side-by-side, they got a Gemini 1 (110W powered speaker) and the Gemini 1-P (250W passive). They balanced the volume between them and the rest of the guys in his band didn't think it was even close to compare. The powered Gemini had, according to him, far and away the best punch and feel. He returned the Gemini 1-P the next day and focused the review on the powered Gemini. Given, I knew of that story before purchasing the Gemini 1-P and my powered Kemper, and for a person like me it isn't the end of the world, but that story is pretty consistent with what I've heard from a number of industry professionals. They much prefer providing their own power, be it through a speaker or a power amp like the Matrix GT1000FX.
For me the powered KPA works fine and I like having the option for the sake of convenience, but I'm sure I'll end up investing in a powered speaker, most likely from Mission Engineering.