The X3L is so configurable, there's almost too many different ways you could do this.
Do you have the "Pilot's Manual" (the thick red book that should've come with your unit)? There's a section in there on configuring your outputs. Basically, you want to take one of your 1/4" line outs to your amp, and use the XLR/studio output to send to FOH. The FOH feed should be configured pre-volume, so when you adjust your onstage volume with the pedal it doesn't mess up what your sound engineer's trying to do out front.
Also, your 1/4" line out has several settings. Use the one for going into an amp/cab. It's supposed to have some eq settings to "balance" the tonality of the cabinet a bit so your models aren't colored coming out of another amp.
EDIT: I honestly can't speak to the amp/cabinet eq, since I run into a HiFi cab, but it at least sounds like a good idea.
