Tim's almost got it. The SBM1 provides a fixed gain and then levels are set post gain by running the signal through a variable resistor (or maybe signal first goes through the variable resistor, and then onto the fixed gain stage, can't quite remember at this point). So run the SBM1 at 10 and the signal effectively doesn't go through the resistor.
It all depends on how your ears hear things, but theoretically it will be best to run the mp2 low and the SBM1 close to wide open. Less signal degradation within the SBM1's variable resistor this way, and less gain applies at the mp2, whose gain stage gets worse at higher levels of gain, at least IMNSHO.
-Todd