Is this an integrated amp, or a system of separate pre-amp and power-amp? Does it happen with the inputs shorted (i.e. the positive and negative connected together, on each input)?
If it's affected by the volume control, then the hum is coming from some stage that is before the volume control (i.e. not the driver / power stage). If it happens when the input is shorted, then it could be noise leaking into the circuit from the outside (due to lack of shielding around sensitive wiring) or it could be heater noise from one of the tubes between the input and volume control. If it does not happen when the input is shorted, then your test is picking up ambient noise from the input.