Here's the easiest way that I've found... and I suppose it may not always work to NOT alter your tone, but it seems to be a pretty clean boost for me.
If you have your base tone dialed in, and as long as you don't have your amp's channel volume max-ed out, you should be able to simply toggle between two different channel volume levels.
Here's how I did it. Get your amp model's channel (base) volume where you like it. Let's say its at "8.2" for example. If you have an open footswitch slot, you can go into the advanced options menu (three lines icon button at top left next to "save"),
- press "controller assign" knob
- make sure 1st knob (parameter) is set to "Ch Vol"
- change 2nd knob (controller) to the open footswitch slot
- "Type" knob should be "latching"
- "Min Value" should be set to your base level (8.2 in this example)
- "Max Value" set it to something high enough to qualify as your "boost" - lets say "9.4"
- Hit the "home" button to exit the options
Now you should be able to hit that switch to toggle back and forth between your low and high levels. You can also go one step further by "long-touch"-ing the boost switch and go in to customize the label for the footswitch. I use "BOOST!"
Obviously, if the channel volume is changing your base tone more than you like, you could try tweaking the "master" control between 2 values instead.
But as others have mentioned, it seems like placing an EQ stomp after the amp and cab and dialing in the level to a good boost level should work as well.