Balance control?

Good point. Our eyesight is not perfectly balanced either. In fact, each eye will also see very slightly different colors that are 'adjusted' by the brain. 

 Good point about the subs too. I only use one, and there is no doubt that it contributes to the left side because of its physical location. 

@frogman good call on the subs. Mine are in opposite placement of each other. One in the front corner, the other is just to the left of my listening position. I tried turning them both off. Imbalance is still present. What if I told you that the imbalance is present on roughly 60-70% of my records. The imbalance causes the vocals to lean slightly left on 60-70% of my records. The other 40% of my records vocal are dead center and don’t require me to use the balance control. I realize this sounds slightly insane but it is what I’m hearing. I’ve had friends confirm they’re hearing the same thing so I know I’m not completely out of my mind. 

I experience the same issue on some of my recordings.  Some simply don’t have a solid and stable center image; very frustrating.  At this point I would take some of those recordings to your friends’ and see if the same thing happens on their systems.  Good luck.

It is unwise to judge channel balance based on “perfect” centering of a vocalist.  You have no idea what the sound engineer was after when he set up the recording session or what sort of mixing was done.  Very often, the vocalist appears to be standing just to the left of center, near to the piano, in a jazz recording.  With the bass, drums, and etc to the right or behind the vocalist.

Balance control is a tricky thing. You have to consider if, for instance, the vocalist was standing slight to the right or left of the mic