Well, it could be an issue with the equipement (amp or pre), but most likely it is due to position and room conditions.
I had a system that exhibited the same issue you mentioned. In my case, it was due to a lot of glass to one side and a large open space on the other side.
A quick and cheap temporary fix would be to try to hang heavy blankets or rugs behind the side that is louder or put up some hard panels on the side that is quieter.
However, the easiest is to adjust the balance from the preamp.
Bob