I can see it from both sides of the board since I play guitar and run sound. The setting the faders the same on the left and right channel is not always correct. Boards have a knob at the top called gain. (I am going to simplify this just a little.) Basically, that is where you set your main volume for each channel. You use the faders to make minor changes in that volume. The gain would have to be set the same to get the same volume at the same fader levels. Even then, that might not always be the case.

Each XLR is a mono signal as well as most of the channels on the board. In order for you to get a stereo signal, you would need to run both the left and right xlrs to the board. You could run them into their own mono channel (most often done and what I prefer doing) or could run them into a stereo channel on the board. It is a little more difficult for the soundman to run a stereo signal because the have to match the sound of the two channels with volume and pan. It is not that difficult, just takes practice and can be a little intimidating for some that has never dealt with stereo signals.

If you are using any effects like the stereo chorus or ping pong delay you are only getting half your signal to the xlr left (or right) if you have the pod panned center and only using 1 xlr out. If you are going to take into account the stereo effects, pan your single effects center on the x3, but send both xlrs. Or you could pan the x3 left or right and only use that xlr out but you would loose the stereo effect.

If you are using mono single tone patches from your x3, pan them hard left (or right) and use the left (or right) xlr out to send that signal to the board. In this way, you are getting 100% of your signal to that xlr and straight to the board.

One thing you might want to discuss with your soundman is the use of 2 xlr channels if you are using the dual tones on a x3. This is especially true if they are running a stereo mix to the house. You could pan tone 1 to the left xlr out and tone 2 to the right xlr out and send each tone to its own channel on the board. That would give your soundman the ability to work with each tone independently. This would be ideal if you had 2 instruments or an instrument and vocal going through the x3. If you had both running into the board on the same channel, turning up the vocals would also turn up the guitar for example. With the vocals panned right and guitar panned left, and using both xlrs out, the soundman could adjust the signals independently. Same thing with the 2 different tones on a dual tone patch.

I am not going to get into panning of live sound to much but, you basically want to give each sound its on space in the mix. If you run everything equally through the left and right, you can easily end up with a muddy mix very quickly. The only thing directly in the center is the main vocals and possibly a keyboard or kick and snare possibly. It all depends on the room you are playing in and the system setup. You just want to give each insterment its own room so it can cut through all the other instruments and be heard instead of sounding like a jumbled mess of sound.