I also have a George Steck Grand Duo-Art (Serial # VRC 69243).
Everything which John Johns described in MMD 00.07.09 is what I have
in my piano.  My piano was rebuilt before I got it.  I don't know when
or if it was re-tubed right.  Since I haven't seen another one like it,
all I can do is report on how my piano works.

This piano doesn't have a soft modulator valve for reducing the vacuum
level to the Duo-Art regulator (same as John Johns).  When putting the
piano into soft mode, it only raises the hammer rail to soften the
playing.

By the way, the soft switch inside the spool box only has one nipple on
it but you describe it having two.  I wonder if you have installed the
switch assembly wrong, causing the Duo-Art On/Off to be mixed up with
the Soft On\Off switch.  The Duo-Art switch has two nipples, one going
to the #2 accompaniment accordion pneumatic and the other going to ??.
I would have to do further investigation to see where it goes.

The soft hole in the tracker bar controls a very hefty pneumatic which
is attached to a large "L" shaped bar found on the left side of the
keyboard area.  This shifts the entire keyboard to the right when
actuated.  This operates just like pressing the foot operated soft
pedal below the piano.  I don't totally agree in the way it operates
but I will not change it from the original operation.  It does its
function even when the piano is playing with the soft switch set ON.
I wonder if John's piano has this same soft pneumatic.

Another oddity that I have seen on this piano is that when the Tempo
lever is moved to the left (below the 10), this opens a pallet value
found on the air motor regulator which shuts off the piano power.  It
doesn't make sense why this was included on the piano from the factory.
The only reason I can guess is that when a roll start having problems
and is tearing, I tend to go for the Tempo , play\reroll, and power
switch levers all at the same time to try to stop the thing before any
more damage to the roll occurs.

Pete Knobloch