[Radioroslyn]

 I would try a couple of simple things. Unplug the power cord and plug it in the other way round. Sometimes the phasing on the AC line going into the set can make a bit of difference. If the have replacement tubes try swapping those. May have some leakage from heater to cathode or cathode the grid which will cause hum. The rectifier and output tubes are suspect here as they have much larger heater than the others. Connect a earth ground wire to the chassis THOUGH A .05 @600v. Be careful there is the possibility(50/50 depending on which way the plug is inserted in to the outlet) of having 120vac across that cap.
Bottom line is that ac/dc sets are at the bottom of the line when they where designed. Most of them have small spkrs and small coupling cap in the audio stages to attenuate the low frequency response (read 60cy here). This is done to minimize the reproduction of hum. Chassis layout can be a cause if the 1st audio amp tube is near the rectifier  it can pickup hum. Wire layout if there are ac wires (read line cord or heater wiring) near the 1st audio stage. Having the chassis floating above earth ground(using AC neutral as ground) can be an issue.
So bottom line is if you have a little bit of hum with the volume control turned all the way down which is easily over come by the audio signal if the volume is turned up a bit I would lose any sleep over it. It's pretty much a design/profit problem.

Terry
Did you get your Motorola tv up and running?? I've got a few 1948/49 Motorolas the 7" jobs.