I worked it out
Sets like the 9R 59 were not designed with much care over low hum. It was assumed that it wouldn't degrade the readability of the limited bandwidth signals it was aimed at. You should expext some, but not so much as to be distracting. There are causes such as a limited amount of HT filtering, unbalanced heaters and also chassis currents from the mounting of the transformer core screws. There are several areas where you could make a series of small improvements.
David (GM4ZNX)