Using Unbalanced cables causes this ground loop hum, try plugging amp directly to wall outlet by it self, everything else plugged into a standard power strip into a different outlet. Also, make sure ALL connections from cartridge clips (examine if any insulation on cartridge leads are missing or torn...the wires are very thin and need to be completely covered to clip) on back are not loose! Make sure there are no lamps/lights that use sliders or dials on the same circuit that will cause buzzing too. Also, keep IC's and spkr cables away from power cables (best u can).
Matt M
Matt M