Answering such questions tends to get replies recommending whatever
the responder owns, as it's hard to 'test drive' loads of different
players in good condition.

For me, the Aeolian Pianola action and the Hupfeld Phonola are the
most responsive systems I come across.  I was brought up on [Aeolian]
Pianolas, and have had the opportunity to use [Hupfeld] Phonolas over
the years and grown to enjoy using them.  Other systems may be quite
good, but these are the best for me.

What matters is a combination of things.  Not just the changing of
volume in reaction to foot accenting, which primarily relates to
reservoir size and the design of the non-linear spring which causes
the dynamic level change, but also the response of the valves to
perforations.

There's little more dispiriting than a player where notes don't do what
you intended; notes that drop out at low playing levels are the worst,
but also sludgy or uneven response and poor repetition and egregious
faults, and also wind motors that stutter.  Most of these faults cause
you instinctively to increase the playing level to keep everything
going, hence you lose musical control.  A Pianola or Phonola in good
state will simply do exactly what you are thinking of, so you focus
solely on the music.

I also find that a direct mechanical sustaining pedal, as opposed to
a pneumatic device, give far greater control over the music, far more
precise and with possibilities of 'half pedalling', etc., that no
on/off pedal control [push button] offers.  All makers sold players
with or without them, according to the specifications of the buyer.

Both Pianola and Phonola systems deliver all the good things.  And you
also have the optional Themodist or Solodant theme accenting devices
that, if you choose to use them on a suitable roll, take them to yet
another musical level.  The Solodant is a bit more controllable because
it directly regulates the accompaniment playing level, while the
Themodist throttles it.

Julian Dyer