I don't have an Amplifi FX100; but typically to get a multi-FX (with no XLR out) into a PA, you'd use:
Main Out Left/Mono --{mono guitar cable}--> direct box --{XLR cable}--> mic channel on the board
If the PA is running in stereo, and you want to use stereo guitar FX, then you'd use:
Main Out Left --{mono guitar cable}--> direct box --{XLR cable}--> mic channel on the board (panned left)
Main Out Right --{mono guitar cable}--> direct box --{XLR cable}--> mic channel on the board (panned right)
Probably don't want to pan the channels hard left and right on the board -- maybe 9:00 and 3:00, or 10:00 and 2:00. Depends on the room layout and speaker position. You don't want someone sitting in front of, say, the left PA speaker to miss half of your sound.
Personally, I never use stereo. It's just one more thing to go wrong...
Mic channels are generally more flexible, because they typically have a trim control (aka gain) with a huge range. The sound guy uses the trim to match your signal level to the board. The stereo inputs on a lot of mixers don't have trim controls; so if you're using a stereo channel and your signal at the board is too hot or too low, there's not much they can do.
If you have your own PA and you can verify that the stereo input channels work OK with your FX rig, then use them; but if you're using house PAs, it's best to stick with mic channels IMHO.
Finally, make sure you configure the main outs on your FX board to Full Range (or similar), not Guitar Amp.
