This may be of some interest to those of you who have their player
piano in a 'live' acoustic environment (hard wood floors, nothing much
to absorb sound).  I found that putting R-13 fiberglass insulation
between the posts on the back of my upright (the same stuff used in 2x4
exterior wall construction) did a marvelous job of taming my too-loud
player.

I will admit that the pneumatics are not perfect and that a complete
rebuild might allow playing at a softer level, but the insulation is
a very inexpensive alternative.  I put the vapor barrier part of the
insulation facing out and covered the whole soundboard.  It's much more
effective than the wool blanket I had back there.  I can't imagine this
would cause the piano any harm but I figure this is the place to
ask.