G'day Ozzie, everything is earthed to the volume pot.
Stacked blend pot wasn't doing much on coil splits and as you heard with the dpdt mini switch there isn't much happening with one of them either, just more noticeable than using blend that came with the pre amp.
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All is good. I hope you get the outcome you want.
(Thanks for the wetsanding advice 1200 levelled up nicely. Nice full strength coat over it and those little holes are gorrrnnnn.)