1 channel ground hum

av15121

I installed a new preamp. I picked up a ground hum in my left low speaker in a 3 way configuration. It was really bad before I did the ground lift. I had the hum in both L and R speakers. When I did the ground switch on the back of the pa2, all hums were eliminated except for this 1 speaker. I tried a different cable. The ground hum is there without the preamp hooked up. I put the old preamp back in. Still have the hum. ????