I have a special fondness for the so-called "triple titles." These are
popular tunes produced in the years after the Aeolian-American merger
where one performance was coded for three reproducing piano systems:
Ampico, Duo-Art and Welte-licensee.
Having owned all three systems, worked on them, and having listened to
them side-by-side in my home, it is fascinating to me how each of the
different systems show their strengths and weaknesses so clearly when
these songs are played in this setting.
In support of John's thinking, I too have noticed that later Duo-Art
titles (post 1930, of which the "triple titles" are) played more weakly
(or perhaps I should say are coded for softer playing) than similar
titles of the earlier decade.
While I never owned one and I've never charted it's expression curve,
I always guessed that these later rolls were coded with lower volumes
because the late Duo-Art (of the 1930s, with the wrap-around tubing)
had an electric roll motor. Given that there was no wind-motor, an
enormous consumer of suction, the suction for the entire stack and
expression mechanism in turn would have greater ability to play evenly
and without droop at even the softest settings. Whereas in earlier
rolls, we see the need for "8"+"4" consistently in the Accompaniment
for even, mezzoforte playing, in the later rolls only "1"+"2" are
consistently called for.
In the absence of the wind motor, the "Play/Don't Play" test on the
test roll is misleading at best. My solution on my instrument was to
regulate according to the test roll and make the playing of the
pre-1929 rolls even and good. And then with the occasional 1930s roll,
to throw on the "Dance" switch which added the "8" accordion to the
Accompaniment and evened out the volume overall.
If you don't have the "Dance" switch you could easily add something
that would achieve the same end without altering the hardware.