The idea that a Duo-Art stack needs to be leaky is simply wrong!
Of course a valve cannot regulate nothing.  But system flow occurs
when one or more stack valves open, and the knife valve then regulates
the flow and suction level instantaneously, perfectly well as intended.
There is absolutely no justification for a constant, unquantifiable,
unpredictable in-leak through the stack.

To get a Duo-Art to play properly the stack must be effectively
airtight, and it is not difficult or impossible to make it so with good
rebuilding techniques.  These include rendering the stack pouches
effectively airtight, too.

People rarely complain of a live pianist playing too loudly -- as loudly
as a reproducing piano.  It's a psychological aspect that has been noted
before.  To do its artists justice a reproducing piano should play as
softly and loudly as a live pianist would normally play it.

Similarly, some like to make their reproducers play inhumanly softly.
Of course they should be able to play very softly without missing,
but softer than a live pianist would play is unnecessary.

Some rolls were simply rather coarsely coded and are loud; some
(especially some British-coded rolls) must always have missed in
certain pianissimo passages on a Duo-Art set up according to certain
test rolls.  Some slight zero-level compromise may be necessary,
but very little on a well-regulated instrument.  Naturally, a
poorly-regulated piano action will anyway prevent even, soft playing
without missing.

Progressive voicing of a reproducing piano is also vital.  The
performances of many supposedly well-rebuilt instruments are marred by
astonishingly poor hammer voicing, which bears no relationship to that
when they were new, and definitely not the uniformly hard, metallic
brash sound liked by modern pianists and rebuilders who know no better.

It's important to get the accordion linkages correct, so that the
response curve is not convex, which results in too few pianissimo
gradations, too much mezzoforte and a rolled-off fortissimo.

And no Duo-Art will ever play softly without missing or play fortissimo
realistically if its pump doesn't pull a high enough suction, or its
unloader (on many British Duo-Arts) is set too low.  A good pump should
easily pull at least 50 inches of water suction.  With enough suction,
the zero level can be reduced significantly without missing, and this
brings down the fortissimo by a multiplier.

Well regulated, voiced and adjusted Duo-Arts do not play too loudly.
There is never any justification for altering the standard accordion
increments to reduce dynamic range.  This will always compromise the
ability of the knife valve to play softly and meet the chord capacity
tests.

One caveat is that appropriate regulator springs are fitted for the
size and dynamic range of the piano.  Aeolian had at least three
"strengths" (rates).  Too strong springs will make the piano come up
too loud too quickly; too soft will restrict the dynamic range and
make it sound anemic.  The original springs are almost certainly
appropriate providing they have not been over-stretched.  But all the
above points should be addressed before suspecting regulator springs
need changing.

Here's a well-voiced, regulated and adjusted Steinway O Duo-Art with
an airtight stack: https://www.youtube.com/watch?v=4jOslQGCgw8 

Patrick Handscombe
Wivenhoe, Essex, UK