I am using the unbalanced 1/4" input sockets that override the Balanced High Z Inputs. Since the Balanced High Z Inputs are overridden (per the Altec 9440A operating instructions), the transformer sockets are out of the circuit. I do not have the transformers installed and I have no jumpers (jumpers are not an option with the 9440A). The first chance that I get, I will remove everything from the rack that ties the chassis of all units together and see if the hum is still there with none of the units connected to the common metal rack. Thanks for your interest bfish.