I too have been plagued with this problem from time to time. It has almost always been solved by speaker placement, toe in and/or distance from the listening position. It is amazing how seemingly insignificant changes in these two factors can cause dramatic shifts in the image. If your system is set up properly and everything is working properly, you should be able to look at the spot where the vocals are coming from, and that spot should be centered. I find it hard to believe that you can trick yourself into thinking it is coming from some place it is not, especially when you are concentrating on it.
Here is a simple way to tell. When I think my image is centered, I turn my head to look left of center and the image comes from the right of where I am looking. Then when I turn my head to look right of center, the image comes from the left of where I am looking. In other words, even when I turn my head, it is still coming from the center. If the image moves left when I look left or to the right when I look right, then the image is indeed off center in that direction.
To see if it is something wrong with the system, move the left speaker to the right and vice versa but leave everything hooked up the way it is. Hopefully your speaker cables are long enough. That way everything that was feeding the right side is now on the left, and vice versa. If the image is still off center in the same direction, then you have a problem with one of the following:
a. toe in
b. distance from the speaker to the sweet spot
c. some sort of interaction with a room irregularity
d. a hearing problem
Here is a simple way to tell. When I think my image is centered, I turn my head to look left of center and the image comes from the right of where I am looking. Then when I turn my head to look right of center, the image comes from the left of where I am looking. In other words, even when I turn my head, it is still coming from the center. If the image moves left when I look left or to the right when I look right, then the image is indeed off center in that direction.
To see if it is something wrong with the system, move the left speaker to the right and vice versa but leave everything hooked up the way it is. Hopefully your speaker cables are long enough. That way everything that was feeding the right side is now on the left, and vice versa. If the image is still off center in the same direction, then you have a problem with one of the following:
a. toe in
b. distance from the speaker to the sweet spot
c. some sort of interaction with a room irregularity
d. a hearing problem