[Mondial]

Yes it definitely seems there is an audio ground loop associated with the low end of the volume control. Probably the biggest problem is that there may be a couple of amps of AC filament current flowing through the chassis which is common grounded to the low level audio.

Are there two separate wires to the filament pins of each tube, or only a single wire and chassis ground to the remaining pin? On the more expensive sets, they ran twisted pairs of wire to the filament pins and grounded the center tap of the filament winding, so there was no filament current through the chassis.

You might try experimenting with where to ground the low end of the volume pot. Try to find a point on the chassis with minimum induced AC hum.