"Are you certain that the two speakers are connected with the same phase, i.e., that + and - are not interchanged in the connections to ONE speaker?
If the two speakers are not connected in phase with each other, some of the results would be vague and diffuse imaging, and weak bass. I'm thinking that in some circumstances a result might also be the channel imbalance you have perceived, and also a perceived reduction in volume (which would be further compounded in one channel by the balance control offset)."
That could be the problem. If you play a recording that normally has a solid center vocal image, wiring the speakers out of phase with each other will make the vocals sound like they are coming from one of the side walls, and not the center. Its very easy to hear. You can't miss it.
Another cause for the channel imbalance could be your room. If you have something like the right speaker near a corner and the left near a free space, one speaker will have to work harder to match the volume of the other one.
Also, if you bought your speakers used, go over all the screws and make sure they are tight. They can loosen up over time.
