[Arran]

 I think it's time to do some voltage checks, starting with the output of the 35Z5 on the input filter capacitor and working your way along checking the plate and screen grid voltage on each tube. One common culprit is the plate resistor on the 12Q7/12SQ7 tube, but if the tube is weak that will also effect the volume. The second IF transformer also has a fixed mica cap inside, a .00015 uf, that is paralleled with the volume control, if those have a habit of going bad that could cause a problem. There is another mica that runs between the plate of the 12Q7 and B- (C11), that could also cause low volume if leaky, but a voltage check will tell.  What is puzzling is why adjusting the trimmers on the IF cans doesn't do anything, if it doesn't make the radio perform better then playing with them should make it worse, or is it just that it does not make it better?
Regards
Arran