On my piano, the "Bass Soft" and "Treble Soft" expression controls
reduce the distance between the hammers and the harp.  In other words,
the hammers are not as far away from the strings when at rest.  The
volume of the key strike is partially a result of the distance in which
the hammers have to gain speed when the key is actuated. 

On my piano I installed a long piece of wood on a hinge mechanism which
is controlled by what we used to call a "choke cable", similar to the
arrangement commonly used for a mandolin rail, so that volume of all
notes can be controlled without holding any buttons down.  This, in my
case, is particularly helpful when playing rolls with tracking problems
which require heavy action on the foot pedals to get the notes to come
out right.  I'm sure that any degree of volume control desired can be
achieved if this method is carried out to the desired degree.

Lee Rothrock