I am sorry, now that it’s not 3 in the morning and I have the FX8 hooked up I see I was wrong. You should use Config tab then Midi PC. However, I do believe that in footswitches tab, you can use 1 button to switch back and forth between two amp channels by using PC on PC off. Do below first before messing with this. You need to save your amp states and a PC location.
And what I mean by PC 0 and so on is PC “Program Change” is what the JVM uses to change its state and 0 is the location of that save program change. The JVM can save from 0 to 127 which is way more then you will ever use.
Now in FX8-Edit, click on config and then Midi PC. That screen will show all 8 scenes with PC and CH. You can select any number from 0 to 127 in each scenes. I am using a Revv Generator 120 so I don’t have the JVM anymore but I am pretty sure it will use the PC and not the CH.
What I would do is plan your amp state out that you want for each voicing. For example,
Clean green, Reverb off, master 1, loop on = 0
Crunch amber, Reverb off, master1, loop on = 1
Crunch red, reverb, master#, loop on = 2
OD green, yada yada yada = 3
OD amber blah blah blah = 4
OD red hold you fucking ears = 5
Now create let’s say Preset A1 name it JVM channels. Activate 6 buttons as scenes and go into config and select Midi PC. In scene 1 PC select 0. Set you amp to clean plus what ever you want on and then hit the program button on the JVM twice so the red light stays on, then press the scene 1 footswitch on the fx8. Now your clean is saved to PC1. Do the same for all the remaining scenes. scene 2 PC select 1 and save crunch amber and so on. Once you save all amp states to 0-5 you will not have to re program your amp again. Now all you have to do when you create a new preset is to select 0-5 meaning which amp channel you want on the scene button. Let’s say you create A2 and name it Van Halen and you only need an over drive channel just select Scene 1 as footswitch 1 and select Scene 1PC = 4
That should do it!! Let me know how you make out.