There is no schematic for the small board with the potentiometers. A guess: the "master volume" potentiometer regulates the general signal level to the board. "High pitch" could mean the level for the two full-range speakers while "bass" rather obviously is the level to the sub-channel.
The bandpass filter is shown to be a fixed filter. I doubt that there is bass and treble regulation as the small board only has few and passive components.
hey, i guess that small plate is the worst design option, option A