I think thr axe fx is not the problem. A large percentage of all the heavy music coming out in the last year or two has one of those things in it somewhere, if not everywhere. Don't get me wrong, you can make them sound bad, but that is user error, not a failing of the unit.
I diagnosed the problem with the mixes early on. The guitar sound is fine, even if it's a little too processed. The trouble is the bass and all the rest of the instruments have the exact same tone dialed in.
The mix would sound huge if the bass had some low end to it and had a different midrange than the guitar. ( especially in the parts where the bass is being played in the same octave as the guitar.
There is no need for any fancy tricks, special processing , amp purchases, a higher sample rate, delays, mid/side processing, multi-band compression, parallel compression, etc...
Simply make the other instruments sound different than the guitar, which they should anyway, and the mix will sound huge just having those guitars panned wide.
If you do have a center guitar, mute it. It will add to the thickness, but it will take away from the stereo spread of the guitars, unless it is playing a different part than the main rhythm.