As tubes age, some mechanical warping takes place (as I understand it, typically the filament) due to the number of heating/cooling cycles. Since these tubes are well used, it is reasonable to guess that's the reason for the modest change in hum level.
Here's a little background: The design goal was 2mV, which is quiet enough for all but the most efficient all-horn speakers. Without the DC filament supply, I was not able to get hum below 5-8mV. If the hum is a problem, an electrolytic capacitor can be installed at the output of the filament supply to reduce it further. We didn't include that in the stock amp in the interest of avoiding capacitors in the filament supply, which some listeners think are responsible for a "flattening" of the sound. I find it to be a very subtle difference, but YMMV - we all hear a little differently.
In the current Stereomour and Kaiju, we have separated the DC filament supply as upgrade kits, since for most speakers even 5mV is good enough to be inaudible.
