[Arran]

Here's another possible culprit for the low volume, I see on the schematic that this set has a pair of audio interstage transformers, it isn't uncommon for the primary of one of those to open up. Check the plate voltage on the 6F6M driver tube, also check for plate voltage on the 86M 1st AF amp. With regard to the interstage between the 6F6M driver tube and the 86M 1st AF amp, there is also a 20,000 ohm resistor in series with the primary of the transformer so check that resistor as well as the continuity of the transformer.
Regards
Arran