My soundstage is shifted a bit to the right. My situation is due to the shape of the room. The balance control works far better than anything else I've tried. Its been the only thing that didn't have any obvious trade offs. Everyone's room and system is different, and nothing will work in every situation. The best thing is trial and error. Marking speaker positions is will help out a lot when moving speakers around.
As stupid as it sounds, some cables have caused my soundstage to shift, and some pre-amps have as well. If the channels aren't prefectly matched volume-wise or if the cables sit on opposite ends of their manufacturing tolerances, and inbalance can occur. Most of the time its due to room acoustics and speaker placement, and not the equipment.
One other thing... Have you tried moving your seat?