I think Dave Saul (the AMICA Technicalities Editor many years ago) once
showed me a sketch of this device.  My recollection is that the pneumatic
porportionately _opposed_ the action of the small expression pneumatics
(2,4,6), and the result is compression of the output stack pressures.  The
"Loud" mode, when engaged, _aided_ the expression pneumatics and expanded
the dynamic range. ... Or maybe I'm thinking of the "Subdue" and
"Brilliant" positions of the later Ampico A.

Robbie Rhodes