To determine if the preamp is the problem, switch the left input for the right and vice versa. To determine if the amp is the problem, switch the right leads for the left leads. To determine if the speakers are the problem, switch them, placing the current left on the right and right on the left. If the problem persists after these tests, the answer lies with your room. If the room is not symmetrical, identical on left and right because of an odd shape, this is a likely culprit. If the furnishings are different on right and left, (i.e., a big couch on one side and a window on the other) this will cause an imbalance. If you can't do anything about the room, use your balance control to shift more signal to the right speaker until both sides sound identically loud.
