Just a couple of observations based on the pictures you posted. That orange twisted pair of wires seems to be the filament heater supply for the tube, and that might be introducing hum around your input jack. It also may be that due to the low cost design they haven't included an artificial center tap for the 6.3 VAC going to the filament. You could try moving the orange pair of wires away from the input jack and see if it helps.