Generally speaking, just like with the tube amps that have been profiled, it is a combination of appropriate selection of guitar/amp/speaker cabinet/microphone placement for the particular genre/song:
You are missing it in what kind of mix? In a recorded mix? In FOH? Onstage?
How are you listening to the KPA?
When you find the secret setting, please let me know! 
I will gladly pay you a fee,
When you find the secret setting, please let me know!
I am currently testing a green boost (annon tube drive) after kemper and it is not changing the sound but in fact is making the lead more warm and with a better dynamic and sustain, need to continue with my tests but first impression is that helped a lot!
I think the secret is in the fingers.
No problems here with my guitars in the mix!
Volume is #1
Louder always sounds better to a point. If you play a KPA through a guitar cab at the same volume as your tube amp you'll get at least close. Many people audition them at polite levels and compare that to loud amps.
For me it's almost exactly like a tube amp but better.
They call it mid boost and it works with everything not just the Kemper
It can be done many different way,
-Some will just add a tube screamer in front, it cuts a lot of bass and consequently the mids are boosted. Many in the heavy metal world over do it and almost every single profile is boosted with some type of a tube screamer, but hey even SRV used it.
With EQ it can be done many ways using Low Pass filters between 5k and 7k and high pass between 50 and 100 or even more depending if the the sound is too bottom heavy.
or you can simply slightly boost a mid frequency with a wide Q and sweep through frequencies between 300 and 1.5k to find the best sounding frequencies. At 300 there's' so much body but you don't want to over it, never boost more than 2-3db. Half a db boost might do the trick. Many prefer cutting rather than boosting so filters seem to be the way to go for many (low pass and High pass will focus the sound without dramatically alter the tone)
Finally, I agree… volume is most important when comparing…
But for me it’s almost like a tube amp, but not quite as good…
It's not a tube/SS/digital thing, it's a frequency thing. You have to set your EQ in a way that the guitar is heard without stepping into the frequency ranges of the vocals and other instruments. If you're truly "cutting through" the mix, you're too loud for the overall mix. You want to be heard, but not overpowering.
My gigging rig (back when I was still playing out) was dialed in to fit in the overall mix of my band perfectly. On its own, though, it didn't sound good at all for home noodling. Not a whole lot of gain, not too much bass or high end, and a strong emphasis on the midrange. It almost sounded like I was playing through an AM radio.
Finally, would you be good enough to email me at [email protected]
I need your help,
thank you,
Keith
PS… It will not allow me to send you a message…
The secret to cutting through a mix is....
... dun dun daaaah...
...Arrangement!
Anything can cut through a mix provided there's space for that thing in the mix in the frequency spectrum at that point in time. That's why typically most guitar you hear on record is massively hi and lo passed and is often even sidechain compressed against the kick drums etc. When it comes to solo time space is made in the mix usually through artful arrangement but also simply through dropping back levels and eqing other instruments to make the room.
If you're playing live you can add hi passing in the kemper to get rid of the mud and use a treble boost to help add a more immediate and urgent feel to the sound (thanks to more treble, which is why e.g. the piccolo sounds so aggressive in an orchestra compared to the flute), but fundamentally you should ensure you're not playing in the same register as other instruments (which is why guitar solos tend to be at the widdly end of the neck), and that the rest of the band doesn't try to fill in all the remaining space.
Give your notes air to breathe. Space, Frequency, Volume.
I also like pure cab, maybe because it removes phasing issues, a little pure cab seem to make the KPA fit better in a mix.
It´s not about tube or solid state oder digital. It´s about EQ. The smoothest, biggest sounding guitar sound with the most expensive boutique gear might not be able to cut through the mix if the EQ is not set properly to your specific acoustic environment, respectively your music genre.
Many engineers in the Rock genres f.e. choose dynamic mics like sm57 or md421, because these mics deliver the midrange punch which might be needed to cut through the mix. You might look up for some profiles which use these kind of mics and I´m sure you will get a step closer to your aim.
You could also search for some mixing tutorial on youtube f.e.. That can be an ear opener.
And don´t listen to people who say the Kemper is not able to cut through the mix or whatever. They just want to stir up trouble.
Good luck!
Most problems of "cutting through the mix" result from the mix itself. As soon as you optimize the sound of the drums and the bass you are 80% done with a good band sound and the guitar sound will fit in naturally.
Nah, just go louder, isn;t that the guitarist way?
Only joking of course..
Another factor( assuming a live setting), is whether you are in a single guitar or dual guitar band. single guitar band is more forgiving because you are less likely to clash with other frequencies, dual guitar is a different matter.
The KPA can sound so full that actually yes you might need to cut some frequency bias as mentioned above.
Nah, just go louder, isn;t that the guitarist way?
Only joking of course..
Another factor( assuming a live setting), is whether you are in a single guitar or dual guitar band. single guitar band is more forgiving because you are less likely to clash with other frequencies, dual guitar is a different matter.
Fact! For me, it's usually one of a few things. Sometimes it's getting the right amp profile. What sounds killer at home sometimes does not blend or contrast with the other guitar. So I find an amp that fits better and then tweak from there. Or maybe a little bit cleaner rig (lower gain.) Sometimes cleaner is reducing the amount of effects I'm using. By amount I mean MIX level or maybe an effect just isn't "cutting" it (pun intended) and I either terminate it or change to something similar.
Haha, no seriously, great thread and lots of useful information here!
When I play live,( if we are using cabinets on stage ) sheer volume through a 4×12 is good medicine! However, I have had to get used to the fact that the powered KPA does not ‘sweeten’ as you crank the volume up, like a tube amplifier ... but getting the speakers moving helps a bit.
Something I just do not understand… almost all of the heavier profiles on rig exchange have no mids…?!
Don’t expect to be heard very well if you are afraid of the middle control…
Maybe This is why some people say the Kemper does not cut in a mix, all these metal guys are sucking the mids out of their tone…
What sounds great, playing by yourself in your basement when your parents are away will not translate very well in a live band situation…
In my first rehearsal with my first band I thought my amp was broken until I discovered the mid knob, that's when I learned about the The Fletcher Munson Curve 
If you play louder than the level that the rig was EQ'd at you will need to pump up the mids to compensate for the Fletcher Munson effect.
It's not a flaw, it's the way our ears work. The person at the desk will take care of it in FOH.
Always makes me laugh when guitarists insist that the engineer not touch the EQ on their guitar. 
