[JohnM]

Your first options are to move where you sit or where your sub sits. If you don't need to sit in the middle of the room to listen, ignore it. If that's an important listening position, try moving your sub. Is the dip present across the width of the room? If so, try placing the sub against the wall half way along the length, if the dip is down the length try placing the sub against the wall half way across the width. You will lose some overall output compared to having the sub in a corner, though. If your room is not rectangular try the sub in each corner to see which gives the smoothest overall response.