Sequential circuits samplers

>>synths/samplers with those filters have one additional VCA for
master control<<

you're right, & it may well have been another curtis chip in some
designs, where cost wasn't too much of an issue... other designs used
a cheaper vca for a programmable master volume, or scaled the vca
envelope volts going to the voice chips, or else didn't bother & just
had a hardware volume control.
I think there are some synths that did all three aswell... :-)

duncan.