I've got a possible solution, and if I'm right it'll cure that "stupid/frustrated" feeling like a charm: hold the mute button down for about five seconds or so. There's a red indicator symbol in the lower right corner that tells you when the main zone is on - if it's not on, it means that the headphone jack is on and the main zone is off. Toggling between the two is done by holding the mute button down for several seconds (about 3 for the front panel, about 5 for the remote control).