Hi Steve:

I am not familiar with the B&W unit in particular, although I know about
them.

The reverse grid current makes me suspect leaky coupling capacitors. I have
repaired a number of Collins 75A4 receivers with leaky mica coupling caps,
which then makes the grid voltage positive. The leakage is quite small and
it is useful to use a scope or vtvm to measure voltages. Seems to be more
common when a solid state rectifier is substituted for the vacuum tube
rectifier.

A sudden change is more likely to be capacitor related than resistor.

Good luck, that is a nice piece of history.

Colin K7FM