Speaker placement/balance

System is Thiel 3.0a's, Moscode 600, Rogue 66.
I'm trying to get a proper balance from the speakers. It could be that the Thiel's are doing this, but if I move my head a couple of inches one way or the other, the balance on the vocals seems to make a sweeping move in the opposite direction (when I move my head to the left, the balance seems to move towards the right). It also seems that the bass is coming with a bit more volume on the left.
Switching speakers seems to make no difference. I've checked for phase. The room is a bit oddly shaped behind the speakers. The left speaker is several feet out from a typical corner, the right speaker has somewhat more open space behind it; in front of a foyer area.
Any suggestions are welcome.