Although in this case the OP needs to adjust the balance as well as volume, do you know if Glenn also made them with a single volume pot and a balance knob?
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I've never seen one, but Glen was very obliging and would build virtually anything you wanted.
I'm not sure what the state of affairs is with Croft these days.
It's something Decco or several other companies might take on.
A good quality stepped attenuator could be used, the original pots were cheap ones from RS Components, Glen fitted a single stereo Alps for me.