The unit is calibrated and sound fantastic!!!
The only problem is the hum .
It's 50hz predominant and depends from output level.
I think it's not heater problem.
This is same issue I mentioned earlier. My problem turned out it was from the heater, which is why I installed DC heaters for both of my units. The leads from the the transformer and any off board wiring to the heater connections need to be twisted very tight, but you also have to make sure you don't pull out the windings from the transformer.