[Arran]

 Sometimes testing resistors is not often good enough, I would replace the plate resistor on the first audio tube. The next thing to look into are the grid cap leads, on the first audio tube and sometimes the first detector tube or RF amplifier tubes, it may also be a grid cap itself, failing solder joint. Dirty tube sockets can also cause problems, although not usually with octals or the old six and seven pin tubes in an AC set. 
Regards
Arran