I recently asked Tech 21 about this exact issue, because I wanted the VTDI cab sim to go to the board but not to my cabinet. Even if the cab sim is off (the button isn't pressed) and you're not hearing it through your cabinet, it is automatically engaged for the xlr out only as soon as a mic cable is plugged into the xlr out. This is accurate -- if you listen through the board while playing and toggle the cab sim button on and off, you'll hear no difference. You can simultaneously use the regular 1/4" out (not the parellel out) to go to your cab and you'll get whatever you have dialed in on the pedal out of your cabinet without the cab sim, assuming the button for it isn't pressed. Naturally this isn't in the manual anywhere, which is why I got in touch with support.