I have been reading this thread, and noticed the symptom takes about an hour to develop. It
seems to me distortion of the audio is occurring.
The grid bias for the 6F6 tubes is developed across resistor 58 (19 and 109 ohms). The other thing
about this, is the three filter capacitors. The polarity of connection should be checked,
as the + of 59 connects to the – of 59A and 57. Also the 19 ohm section of that
pair ( part 58) should be checked as well as the 109 ohm part. The mid tap
of this pair provides bias to the 6K5 audio amp, as well as a suppressor bias to the
1st RF amp. The AVC, and the operating point of the 6K5, is controlled too, both can
affect the sound.
The other thing that might help is to connect your meter between chassis ground and
the end of the 109 ohm section of part 58 that goes to yellow green wire on the power transformer.
While the radio is playing see if that voltage changes as the sound gets distorted. You
could also measure the midpoint of that resistor pair (part 58) as well.
Some causes could be gassy 6F6 tubes, or leaky capacitors to grids, but I think you already changed those.
That output stage in the radio depends on the screen of the top 6F6 providing drive for the grid
of the bottom 6F6. This makes resistor 52 quite critical.