FWIW, over the 4 years or so I've owned a unpowered Kemper, I have tried every power amp I can lay my hands on with it. I tried the Kemper and external amps with both FRFRs and standard guitar cabs and speakers. Here's a brief list of at least the ones I can remember:
McIntosh MC30
Dynaco ST70
Multiple Marshall tube amps power stages (Haze 40, VIntage Modern 100W)
Behringer NU1000DSP rack class D power amp
Marshall 8008 Valvestate rack power amp
Crate PowerBlock (using the RCA input with no speaker emulation)
Multiple class D power amp modules with different chip sets (TI TPA3116 in regular and bridged mode, Tripath)
Peavey Bandit Transtube power amp stage
ICEPower 50ASX2BTL module (same as in SD Powerstage 170)
Sure AA-AB31241 600W class D power amp module
In most cases, the solid state amp modules had a similar feel in the lower mids and bass, kind of a plastic fake sensation. The tube amps seemed a bit mushy and indistinct, for the lack of a better term. Very distracting and disappointing in most cases, and I really didn't enjoy the experience due to this coloration. I was not happy with the majority of the amps, even though I was very pleased with the direct tones of the Kemper.
But the single amp I've tried so far that was significantly better than all others (tube or solid state) in tone and feel is the ICEPower module. The distracting tone and feel in the lower mids and bass is absent with that amp, and I found I could lose the sensation I was playing a profiler and get lost in the tone. This has endured over the last year I have used the ICEPower. IMHO there really isn't any other choice. The next one I plan to try is the ICEPower 200ASC module, promises to be a bit easier to integrate than the 50ASX2BTL.
But overall, if I had to do it all over again, I would have bought a powered Kemper. After 4 years of Kemper use, the internal power amp is essential for my needs.