However if this is a passive mixer, with the switch there, then as you put more or fewer inputs on one group, the mix-loss and level will change. To keep loss constant, unused mix resistors should be grounded (actually, grounded through a resistor similar to the pan-pot impedance). That won't be a standard switch.
Reconsider your grounding arrangements carefully and you can probably improve it further. Poor ground bussing can degrade your isolation by many decibels.
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