Is it 120 or 60 hz hum? If it's 120 hz it would suggest something power supply related since the power supply almost certainly uses full wave rectification which results in 120 hz ripple.
60 hz hum could be pick up from power lines, ground loop problems, heater to cathode leakage in tubes (unlikely since you've tested them and tried substitutes), or any of a number of other things.
60 hz hum could be pick up from power lines, ground loop problems, heater to cathode leakage in tubes (unlikely since you've tested them and tried substitutes), or any of a number of other things.