It could be any of the above. I'd check your interconnects and make sure you are properly grounded. You can even open things up and look for a loose contact. If that does not help then eliminate items by substituting things.
For example wire your power amp in reverse (left to right speaker and right to left speaker) and see if it moves to the left speaker. If it does not then you can eliminate your power amp as the problem.
For example wire your power amp in reverse (left to right speaker and right to left speaker) and see if it moves to the left speaker. If it does not then you can eliminate your power amp as the problem.