Could you use an expression pedal for this?
Yeah, actually. I do, anyway. And, I do it in a way that's really simple and deosn't involving any "minimum" or "maximum" level numbers being set in the Axe FX...
There are often many ways to accomplish certain things with this device. One thing that I needed to figure out early on (after getting my AXE FX) was the following: I use mine live. Sometimes at my cover band's shows there is no sound-man (or a clueless sound man). There is another guitarst in the band with me & we share lead parts throughout the gig. We play a wide range of styles & sounds. I needed to find the best way to have access to a universal "solo" effect, that is to make the overall volume of my rig louder - but, just during the guitar solos (or critical lead parts) within a given song & then be instantly capable of switching back to the normal volume. I didn't want to have to program a volume or gain boost into every one of my patches. I use LOTS of patches live, which often change as time goes on. For a long time, I was also using a Boss GT-8 as a MIDI floor controller until I was eventually able to buy my MFC-101.
Anyway, the best solution that I came up with (and I still do this NOW) is to use a Roland EV-5 Expression pedal. I simply plug the Roland EV-5 into the "Pedal 1" 1/4" jack on the back of my AXE FX. I also assign this "Pedal 1" jack to control master volume globally. The Roland EV-5 is being used as an external volume pedal. This particular pedal has a wheel on the side of it that you can turn to set your minimum volume level (from 0 to 10). I set this wheel at about "8". So, when the pedal is all the way back, it's slightly quieter than it is when it's all the way forward. It doesn't affect sound, simply overall volume. I keep it back for most of the show & I push it all the way forward for guitar solos or parts where I need to be louder. It works fantastic! It doesn't alter my tone 1 bit. I have another guitarist in the band and I can now be heard over him perfectly when it's my time to solo. If we have no sound man, it's a must. When we DO have a sound man, he doesn't have to do anything for me. He never has to pay attention to when I'm doing a lead or adjust the faders on the board. It's the best solution that I had come up with early on, especially given the limitations of the Boss GT-8 that I was using as a MIDI control device. And, it worked so well, I haven't bothered to adjust it into stomp form, yet. It keeps me from having to remember to program a volume boost of some kind into each of my MANY patches that I use live. Finally, I can easily adjust how dramatic the volume difference is on the fly by simply scrolling that little wheel on the side of the pedal. It never varies much. Again, I'm usually around "8", but occasionally I've made the volume boost more dramatic by setting it just under 8, or more subtle by setting it just over 8. And, the fact that it's an expression pedal let's me boost "half way" or any other amount in between the 8 and 10.
The advantage (I would think) of doing it my way vs. the way other people may be approaching it (by boosting mids or something) is that using the pedal as I do has absolutely ZERO effect on the sound of the patch. Using an EQ to boost often concerns me that the tone will be shaped slightly different once the boost is turned on. This was another concern of mine when I was pondering the possibilities.
Again, with the pedal method that I'm using, there's also some play in the middle. If I only push the expression pedal half-way forward, I only get half of my volume boost. I can go as subtle as I want with real time foot control. If you step on a button for a boost, it's "all or nothing" and only one volume difference.
There's also the issue with dealing with local soundmen. Most I've worked with in local clubs & bars are either clueless, indifferent, unfamiliar with what you're going to be doing, not paying attention, deaf, or drunk. And, when they DO expect a solo coming and try to boost your volume, they often push the fader up late, push it too far (or not enough), and don't bring the volume back down after your solo on time and/or back to the rhythm volume you were previously at. This technique removes their hijinx and incompetence from the equation.
I highly recommend trying this as an option.
Also... With the pedal method that I use, (regardless of what patch I switch over to) I always know if the volume is at the slightly quieter "rhythm" level or the boosted "lead" level by simply looking down and seeing which position the pedal is physically in.
Some of the techniques described here and elsewhere over the years are probably also very good, as well. but, this method has always worked out great for me and I haven't yet thought that I needed to do anything different.
