I think it's enough, with a bit of mental effort. The plugs are to shape the room response (frequency) and we now know from the photos they are implemented as a simple RC network.

It's a long time since I last did this, but a few minutes with Horowitz and Hill should be enough to work out what the circuit must be.

Prod me if I don't post an answer in the next few days ...

- Zygmunt

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