Great build! One question, the gain is calculated on the number of inputs. But what happens if you connect less than 8? Is that's why the 100K are in series with the pots?
Gain required is calculated by number of inputs because inputs still feel ground though the pots even when no signal source is connected. This connection to ground through the pots also means the output level of the mixer is independent of the number of inputs connected. The 100K resistors are to limit the amount of signal one input can back feed into another.
