If the bias diode is two diodes in series, why does the parts list call it a varistor? It's a funky-looking thing that looks like one of those connectors you put at the end of a wire to connect it to a screw. Don't think I'd likely find another one. It's physically screwed to the heatsink the outputs use.
Has technology evolved so that a momentary crossing of speaker wires could not destroy expensive output transistors? You see all sorts amps with "protection" circuits in them nowadays and lots of amp module IC's which boast both thermal and overcurrent resettable protection...
I measured D405 vs D406 in circuit with no power applied and the value, although useless quantitatively, was the same for both channels. I'll check it in operation when I get all the resistors replaced (and Q417).
Bonus for me: the one transistor used in that amp which I've got in stock is 2SC945, which matches Q417 the bias transistor. I like BD Enterprises which you directed me to, they seem to stock all sorts of transistors nobody else does, or their search facility immediately suggests an alternative if they don't have the original.
Dave