I use my ZVEX Mastotron Fuzz in front of the Axe-Fx. That's it. I don't feel like the fuzzes in the Axe-Fx are 100% there, yet.
I'm currently working on duplicating my most-often used effects so that I can leave them in the gear-pr0n closet and just use the Axe-FX. Some things have been pretty easy to dial in in order to get into the ballpark of replicating the pedal.
One thing in particular has been rather vexing: The Prescription Electronics Experience Pedal.
It is basically a Foxx Tone Machine with an added (and quasi-usable/predictable) "Swell" function. I'm not interested in the "Swell" aspect as that is easy to do in the Axe and the Axe is better at it.
I want to duplicate 4 modes:
1) Trebly Fuzz. Similar in sound to a Fuzz Face
1a) Trebly Fuzz (same settings) with the Octave engaged
2) Very Bass-y Fuzz. Closer to a Big Muff Pi, but not quite that
2a) Very Bass-y Fuzz with Octave Engaged.
It took me 4 hours to dial in #1 and then another hour to get 1a. This is the first time that I ever tried to replicate one of my distortion pedals and that added to the needed amount of time.
I spent much more time, maybe 8 hours, programming #2. I want to keep the usage of blocks to a minimum so that I can easily share these. I got nowhere with the Fuzz Face Model and closer with the Rat. I then went with the Muff Pi model and that was much closer. I had to get the last 10% by using Tone Matching and this worked rather well. #2 is now 2 blocks.
I've been farting around with 2a for a couple of days now and the results have been unsatisfactory. I really like having 1 and 1a set up as X/Y in a block. It's just like the real pedal in use. I'm having to engage 2a via a Scene change.
I'm currently traveling and am at a train station, in Geneva, as I type this. I'll be back at it in three days' time, and may appeal to the community in order to help improve the result.
I definitely hold my skills to be responsible at this point. I don't think that it is the Axe that can be viewed as being at fault; yet.
Next up is a Ram's Head Big Muff Pi and then a Korg Mr. Multi.