I don't know a way to dial all fx down as a sort of master fx return channel setting, but it's not hard to assign the mix parameters on many fx to a pedal.
However you can't do this with some floor type fx like distortions and compressors. Compressors can be dialed down in threshold settings. Distortions can be reduced, but they will still color the sound if they're in the chain and active - this is something I would love to see L6 change, with an actual mix control or a bypass when the setting goes to, say, minus zero. This can be done on the Axe FX.
I have discovered a very workable solution for this problem with distortion pedals. I insert a pan pedal early in the chain, followed by two paths, one with the dist pedal and one without. I move the amp to after the mixer, panned center, so it will receive equal amounts of whichever channel has signal fed to it by the panning.
This gives me the ability to go from a clean (or cleanish) lower vol rhythm sound to a louder, more distorted sound simply by depressing the pedal, and it also gives me varying amounts of distortion and volume in the in-between positions. This is a huge advantage for me. I hate looking down at the board every time I want to solo or throw in a line or louder part while comping (my foot easily finds the control pedal without looking). As a singer, I hate all that pedal dancing while I'm trying to entertain. Even when I'm not singing and just playing with an ensemble, the ability to easily control my volume and sound with just the pedal is a facility I love.
I should prolly do a vid on this to show how powerful this setup is, and how easy it is to implement.
