So preface this: it's all entirely my opinion. You may disagree and that's cool, would actually welcome differing opinions. But lets keep it friendly, don't want it all going The Gear Page over here!
A lot of this will be cribbed from my posts elsewhere, but I think there are some worthwhile insights.
I have a Diezel D-Moll valve amp, an Egnater Tourmaster 4x12 with V30's, an ENGL Vertical 2x12 with V30's, a range of pedals also. I have been comparing the Axe FX II and the Kemper for the past few days. I jumped straight to the 2.6 beta and the Axe is running the latest 15.07 firmware. So here are some of my thoughts:
The Kemper tends to sound more bassy and full than the Axe FX, with a more pleasing resonant character. To me it feels closer to the experience of having a cab mic'd up in a live room, where you're monitoring it from the control room. The Axe FX has this rather abrasive nastiness on palm-mutes that is really difficult to dial out, and with my setup is most noticable on open notes and a low power-chord at the 10th fret. It doesn't quite resemble the control room monitoring experience as well as the Kemper. It doesn't sound bad, but... just different, and from a subjective experience, I prefer the Kemper.
By default the Axe models generally seem treble heavy and kind of distant, even when using impulse responses taken where the microphone was only an inch away from the speaker cone. So it takes a fair amount of work to dial them in. Some of the default presence and resonance settings are not very good, and (having had this wish before I got the Kemper) it often makes me wish I could save default setups for each amp model, so that any time I switch to it I get the sound that I want.
Now... I've said before that I got the Axe FX to sound almost indistinguishable to my real Diezel amp through a real guitar cab. This is still true. If you take just an amp block, route the Axe into the FX return of the Diezel and then dial the EQ, presence, and depth (resonance) in... you get something very close to a real amp experience.
If you do this with the Kemper, it sounds like a modeller but louder. It doesn't really sound like a real tube amp... you lose something when you disable the Kempers cabinet section. Even when you use direct profiles (I got some off the rig exchange).
So ... all of this leads me to a few things: I strongly suspect I don't bond well with impulse responses. In actual fact, I think I really dislike them. They make every amp sound distant and unrefined. Even the IR's I've taken of my own cabs suffer from this. The stock Axe FX cabs especially suffer from this. The only IR's I've used that I liked were the GuitarHack ones... but they too suffer from it.
The Kemper is doing something different when it comes to the cabinet modelling. I don't know what exactly, but it sounds much much more realistic to me. If you have a friend play some low tuned power chord palm muted chugs through a real amp and you then move your ear over the speaker cone... you'll hear a massive world of tones available to you, and you'll hear the characteristic low-end thump that the Kemper is producing quite nicely actually. The Axe FX does not do this.
The Axe has really good amplifier models; preamp and poweramp. But I do not like the cabs. Not when compared to the real thing and not when compared to the Kemper.
The Kemper has really nice amplifier profiling capabilities, and whilst it isn't always 100% you can get to 90% of the original tone quite easily. The cab modelling just sounds more realistic to me. It has that trouser flap quality that the Axe is lacking.
So here is where I stand at the moment:
Overall I prefer a real valve amp into a real cab. That is (and probably always will be) the benchmark.
The Kemper I prefer for direct recording.
The Axe FX II I prefer for running into a poweramp and cab, and using as an amp head in its own right.
This could all be honeymoon period stuff of course. My intention is to keep all three solutions for the foreseeable future. We are most likely moving house before the end of the year, and I will not be able to crank a valve amp in our new flat. So Axe FX II and Kemper are going to be lifesavers in that respect.
I do want to figure out if I can get closer to the real amp with the Kemper. That's kind of the next thing here, but right now it's more than usable. It's very enjoyable to play, and requires almost no tweaking at all.
I have a feeling that impulse responses may be the bane of all my woes when it comes to the Axe. As you say... they're static and movement is simulated. Essentially the Axe FX is faking speaker movement and resonance by changing the outgoing EQ curve and poweramp dynamics... the IR is then just acting as a filter. I think the Kemper is actually changing it's built in cabinet model. It will have some internal filtering parameters and dynamic parameters, and they will be set and the min-max curves will be changed based on the information being picked up by the microphone. Okay... that's totally a guess, but seems logical. So what you end up with is a set of parameters given values that correspond to the real world observations the Kemper is making, and then the rest of it is picked up by the dynamic speaker models built within the box. Again.... guessing.
One thing I can say is that when downloading Axe FX II presets, I always
felt like they weren't really usable and required lots of reverse
engineering in order to get them to sound good with my guitar and
playing style. I don't feel that way with the Kemper. I've downloaded
probably about 80 profiles so far and the "transferability" is really
consistently high.
There is a user called 'r.u.sirius' who has
profiled the Diezel VH4, Herbert, and Einstein and his profiles are
really really good.
There is still potential in the future for me to move over to the Axe FX for live performance. But I've never quite managed to get the simplicity that I'm after. My rig is really damn simple with the D-Moll and a few pedals. Plus I would need a poweramp of some kind still, and that is a rabbit hole that I'm rather keen to avoid right now. I need to stop fucking around with gear and crack on with writing music for a change!
Clips:
https://www.dropbox.com/s/azdecwhzkgofgql/D-Moll_A.wav?dl=0 (Kemper)
https://www.dropbox.com/s/i1i225wq1oiwqey/D-Moll_B.wav?dl=0 (Real D-Moll + cab + microphone)
https://dl.dropboxusercontent.…os/PreampComparison/1.wav (Kemper)
https://dl.dropboxusercontent.…os/PreampComparison/2.wav (Axe FX II)
https://dl.dropboxusercontent.com/u/630473/GearDemos/PreampComparison/Track 1(2).wav (Kemper)
https://dl.dropboxusercontent.com/u/630473/GearDemos/PreampComparison/Track 2(2).wav (Axe FX II)
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If anyone has any tips on the best way to use the Kemper into the return of a valve amp, I'd be interested to hear. Also after-market poweramps, as my model is not a powerhead.