This discussion seems to be full of subjective perceptions and descriptions (thump, pressure, compression vs. "open" dynamic sound, etc.), also many Apple to Oranges comparisons (Kemper Profile vs. Axe-Fx Preset -> different frequency responses; FRFR vs. guitar cabinet, etc.).
Cliff mentioned some suggestion to get into more objective territory (TMA the Kemper) and this is a way to get obejctive comparisons...
That said, i was very sceptical and had many issues for myself in these categories in the past, but always tried to figure it out, where it comes from by doing extensive A/B comparisons with many different rigs / signal chains. Just refering to this
thread, as example... where i learned something about frequency response and the speaker page, also with the help from Cliff, he kindly chimed in... and this was on FW13, if i remember right....
I honestly think it's the power amp sim. This weekend I experimented with running my Axe Fx into the fx return on my two Engl Gigmasters, (turned off amp sim and cab sim) and the tube amp feel was back. Considering getting rid of my CLRs and just running that way from now on.
I doubt it. Use a solid-state power amp and plug the Engl speakers into that then you'll have a real test. Until then it's just speculation and uncontrolled tests.
In the thread i mentioned above i had the same impression, my Koch`s Tube Amp gives me a "3D" and "thump", "compression" i could`nt get out of the Axe-Fx Amp Sim respectively a SS Amp, connected to the same (!) guitar cabinet. At the end of the day it WAS in fact 1) a frequency issue. After a TMA (clone the Koch into the Axe-Fx) & dialing in the "impedance curve" speaker page correctly to the actual cabinet, the sound and pressure of the Axe-Fx was as good, as with the real Tube Amp. The "3D" sound (impression, that the sound did not gluing at the speakers membrane,when playing through the Axe-Fx) was gone, after FW14... if i remember correct.
I also had a Kemper for three weeks for a test and i compare both machines extensiveley. And yes - oftenly i had the impression, that the pressure and feel the Kemper do was on some more cleanish profiles better than with the Axe-Fx. But could NOT verify this "first" impression, after re-adjusting and tweak some Axe-Fx Presets. The Axe-Fx can do it ALL! And in the meanwhile better with more in-deep control of certain parameters.
We did at the Axe-Fest 2014 a fair comparison between the Axe-Fx+SS State power amp (Matrix GT1000) and a damn good Plexi derivate, both fairly connected to the identical speaker cabinet (Diezel 212, Celestion G12K100 loaded).
Note the Axe-Fx Input LEDs from minute 2.00 on ...
OFF = guitar -> Lehle A/B switch -> Reußenzehn Plexi Tube Amp -> Diezel cabinet Speaker 1
ON = guitar -> Lehle A/B switch -> Axe-Fx Plexi100W High -> Matrix GT1000 -> Diezel cabinet Speaker 2
The Axe-Fx was SPOT on! With a solid state amp and a Sim, we just tweak
nothing, but Presence to ten! So: No tonematch, no exorbitant tweaking!
You may say now: Yes, but this is not a cleanish sound, i can`t find the "compression" punch and chimey "3D" i am looking for ... I did at the Axe fest exactly the same experiment set-up also with a actual 70ies Fender Super Reverb (stunning clean sound!!!!!), connection the Matrix to the speaker (4x P10R) of that combo and again the same with a Vox Ac30 blonde, loaded with Bloedogs. The owners of those Amps were stunning, how close the Axe-Fx was! Especially for those "3D", Punsh, compression, Amp-in-the room experience, blah blah ....
The Axe-Fx IS superior for what it does and it is designed for! This is ... and this is my personal opinion - a fact. Persiod. A fact, based on comparisons like these ...