I think in 90% of all cases, the KPA will do the job very well. But if you are in the 10% ballpark, nothing beats a full tube amp + 4x12 cab.
I enjoy a couple of additional benefits of the KPA, which others may not need.
I've owned some decent tube amps but embarrassingly, I've never been great at dialing in that killer tone. Someone else walks up to my amp, turns some knobs, it magically sounds awesome, and I just stare at them like a caveman looking at a Bic lighter in fear and wonder.
In a similar fashion, unless you're doing a small enough gig where you're not going through FOH (or don't do studio work), microphones are involved. Fortunately I have fewer deficiencies in this area as small moves have major consequences, but it's another part of what the audience hears.
So, part of what I paid for was a great hardware concept (profiling versus modeling) with a very solid build quality. However, equally important to me was the ability to find profiles others created that already have the tone dialed in and the mics properly placed so I can just find the sound I want and play guitar.
but I couldn’t for the world get a true three- dimensional tone, with depth, balls and precision when using the Kemper through a power amp into my cab ( a Marshall 1960b) or via FRFR (tried alto’s, DXR 10’s).
I started with the powered toaster going to my 1960a. It sounded great. But somewhat different. Ultimately I enjoyed what was coming out of the studio reference monitors so much I got an FRFR to reproduce that level of detail. Again, sounds great. But somewhat different than a real tube amp (with someone other than me twisting the knobs).
I'm adjusting to the differences and I haven't been dissatisfied with my sound since the Kemper landed on my doorstep. Quite the contrary, regardless of what it says about my limited tone skills, my guitar has never sounded better.
However, there's also a caveat to all this. I've never done the rich and famous, play the big stages thing. Best I've ever done is large rock rooms in Long Island, but I just play cover band stuff. And the problem with cover gigs that recording artists don't have to fight is that you're expected to sound "just like the record" when you're playing a night of wildly different guitarists with different styles through different gear. No matter what I've used, it's always been the art of compromise. Or, to use a technical term, "close enough for rock and roll."
For years my solution was a Voodu Valve pre -> power amp -> cab. It has a tube plus digital fx, so I could dial in credible tones, then hit a midi pc and be on a completely different planet. It did a pretty good job, but all sounds were still a compromise versus using the exact same amps and pedals that the recording artist used.
So, I come to the party already willing to compromise a bit in some areas in the name of practicality, because I'm subject to the rules of cover bands. In that regard, the Kemper is the closest thing to a no compromise solution I've ever heard. 100% accurate? Not usually, but still a really, really good sound.
That said, if I was doing an originals gig, tribute band, or any situation where I could take one amp, dial in the sound and roll with it all night, the subtle extras that a real amp offer might win the day so I can certainly dig it. I mean, you know, if I ever actually learned how to dial in a decent tone.