[Heathkit] SB-220 Restoration Initiative

[jack m]

I recently acquired a "stock" (unmolested/unmodified)  supposedly early production SB-220 amplifier which I plan to disassemble, clean up and restore for my personal collection and operation with my paired SB-301/401.
 
I have been advised by others that there are several key modifications that should be performed to ensure reliability and reduce the chances for expensive damage in the event of certain tube or component failures.  Grid resistors and different plate cap chokes have been mentioned, as well as bi-directional meter diodes.  So far, I don't have any details, and I am just starting my research.
 
I have restored two SB-200's so far during the past 10 years, but this is my first pass at a '220.
 
What are the suggested and "aficionado-approved" or collector-recognized modifications for this model, and where can I obtain the technical documentation on these?
 
Thanks - Jack  W3RU