[Mondial]

As Ron said, this is probably as low as the hum gets. I have the same 90 lowboy and there is always a slight hum in the background.

Most likely the inherent background hum is caused by the directly heated filament in the output tube. Regardless of whether you have the model with the PP 45 tubes or the PP or single 47's, they both have the same type of filament heated by AC. The reversing polarity AC filament voltage modulates the bias voltage between the filament and control grid, effectively adding a slight hum signal to the input.

Ideally, if everything is perfectly balanced, the hum should cancel, but rarely is this the case. Radios like the 37-116 with directly heated triode outputs have a hum balance pot to null out the filament induced hum. It's the worst with a single 47 because there is some hum cancellation with a push pull output stage. Also the 47 is a more sensitive tube than the 45 as it requires much less input signal for the same output, so the filament hum will be more noticeable.

The ultimate solution was the introduction of indirectly heated cathode tubes like the 59, 42 and 2A5. Radios with these tubes do not suffer from this effect and have a lower inherent hum level.