It can be the result of everything you have suggested, although I doubt very much that it has anything to do with your wiring. Unless the environment around each speaker is dramatically different I would look to your electronics as the likely culprit. I once had a receiver that was biased to the right when everything was set flat. The only solution was the balance knob. Can you give us some examples of the music you are listening to? I am sure that someone in this forum will be able to listen and tell you how it sounds on their system.
