I have taken your advice. Thank you.
I have found a used SRM-1 pro Mk2 PP version C.
This was the last of this model maybe... Perhaps you can correct me here.
Now for a question. Would it be fairly straightforward to make the bias voltage adjustable?
What would be a good way to achieve this? A separate box perhaps, a separate supply I mean.
I say this because it seems to be something that accomplished DIYers in this thread are doing when testing their creations.
Yes that's the last version of that amp.
Another option for variable bias supply is to get one of the proportional high voltage DC-to-DC supplies from a place like EMCO. They can output a voltage high enough for speaker ESLs. Read the spec sheet to check the input voltage, but I think it's something like 0-15V.