So far, 5 of us have done this and it works like a charm

8BN8 rotten tube, bias diode ages, AGC volts change,
S-meter wanders, Rx gain wanders plus low audio...

8BN8 out, 1N914 diodes in, 12AT7 in, now 2 stages
of audio to 6AQ5.
Sounds just like old time big audio BA Rxs!
Super great audio, particularily on AM, which
was missing before.
Bias, AGC and S-meter rock solid.

Here's the mod...at 8BN8 socket, now 12AT7

1. Noise Limiter diode.
Unsolder the 4 noise limiter diode wires.
Cut off 2 bare wires at grommets on vert NL board.
Transfer the 2 shielded wires to these grommets.
Observing polarity, solder 1N914 across the 2 grommets.

2. Bias diode.
Unhook 1 bare bias diode wire and cut off at bias board grommet.
Remove the 100K and 12K. Shorten leads and hook the 100K from the
same 6X4 pin 6 to unused 6X4 pin 2. 12K from pin 2 to ground.
Observing polarity, hook 1N914 from pin 2 to unused pin 5 and
new wire from pin 5 to bias grommet on vert bias board.
Resolder all other 6X4 pins for good measure.

3. Audio -
12AT7 triodes are pins 1,2,3 and 6,7,8.
This hooks the original AF input to grid pin 2 of the first triode.
Then out of plate pin 1 of the first triode and in to grid pin 7
of the second triode.
Then out of plate pin 6 of the second triode to the 6AQ5.
We have not disturbed the original AF circuit values,
just added a gain stage in between.
Unhook 330K/.005 from pin 7 and move out of way.
Unhook bare grid wire from pin 8 and cut at grid grommet.
Move bare cathode wire from pin 9 to 12AT7 cathode pin 8.
Reroute 2 fil wires to fil pin 9 and gnd fil pins 4 and 5.
Rehook 330K/.005 to pin 6.
Dress and insulate new 330K. Hook from 6AQ5 pin 6 to 12AT7
plate pin 1. New .005 from here to 12AT7 grid pin 7.
New 470K from 12AT7 grid pin 7 to gnd.
New 1K from 12AT7 cathode pin 3 to gnd.
New bare wire from original grid grommet to 12AT7 grid pin 2.

4.
***With VTVM, confirm around -14V to -18V bias***
***Reset tube bias and S-meter per book***
Done.

Try it this way first. If its too much AF you can tailor
the AF Gain setting with a resistive voltage divider at
pin 7 and tailor the freq response some with the size of
the .005 coupling cap feeding grid pin 7.

Best John - Always good in QRZ