This is something I got from a former rebuilder friend from another
forum along with my response.  I thought he said it very well.

Here is the item followed by a source footnote and my response.
It has been forwarded with his permission.

 - - -

  Currently at auction on eBay are Duo-Art "replacement" valve plates.
These are beautifully made and are sold as a replacement for "those
notorious cross valve seats that always leak".  That is the good news...

  However, I have restored hundreds or thousands of cross valves and
had no problems.  There is an issue with replacing the valve seats that
many do not understand and that it is the relationship between the air
flow and "pluck" of the valve.  Pluck is the resistance to activation
from the "off" (or closed) position.

  At the reduced valve gaps utilized by player stack valves, flow
becomes primarily dependent on orifice perimeter.  Under these
conditions, an orifice with a multi-faceted perimeter (such as a star
or a cross) will have greater flow than a circular orifice.  At such
small valve travel (gap), orifice area is less important to flow than
orifice perimeter.

  However, in player stack valves, the total area of an orifice is
a major factor in pluck because it determines the area of the valve's
surfaced exposed to vacuum.  In the same application, the total area
of an orifice becomes a limiting factor in flow only when its area is
exceeded by that created by the dimensions of the valve gap and
perimeter multiplied.

  When utilized in player stack valves, a circular orifice yields
_minimum_ perimeter (i.e., flow) and _maximum_ area (i.e., vacuum
resistance).  It is the worst possible of shapes.

  In the same application, a cross-shaped orifice provides _more_
perimeter and _less_ area.  It has a much better perimeter-to-area
ratio.

Player stack valves employing seats with circular orifices are
perfectly acceptable, as is, in most non-critical applications.  Yet,
the demanding repetition requirements of reproducing pianos dictate
that such valves be used in conjunction with primary valves or variable
bleeds (e.g., Ampico's model B ball-check-type bleed) if optimum
performance is to be attained.  And, indeed, several manufacturers of
reproducing actions chose those routes.

  Aeolian's method of optimizing the performance of a premium single-valve
system for its Duo-Art mechanism was to equip it with the cross-type
valve plate which maximizes flow and minimizes pluck.  The only possible
downsides of this design might have been the critical tooling requirements
of the plate and the need for a highly skilled and patient workforce in
assembly.

  Many complaints of the replacement seats are that the pianos tend
to play too loud.  This is just one of several problems caused by
eliminating original cross valve plates.

  Proper replacement valve facing leather, careful cleaning, not
bending them during removal, plus sealing the bottom of your valve
wells with shellac like Aeolian did, will result in superior Duo-Art
valve work and greater stability by your Duo-Art at lower vacuum
levels.  This equals better (original factory) performance.

  It is true that Aeolian went to the round valve plate, but stayed
with the cross valve in the Steinway and high end pianos.  The less
expensive pianos used the round plates and there were different sizes
of them depending on the model and year.  Also, keep in mind that when
the valve plates got smaller, so did the valve itself.  It is a delicate
balancing act.

  There is definitely a use for the new valve seats, but it will not
improve performance by simply using them as a Band-Aid or panacea to
circumvent proper restoration.

  Restoration?  Do it the way the factory did and it will function like
new.

  Robin

  PS.  The cross valves don't always leak!  In fact they are extremely
reliable and function beautifully.

  Info included from AMICA: Thanks to the AMICA Bulletin too!
  Sept-Oct 1999 by Jeffrey Morgan, "Wondrous Cross Revisited".

 - - -

My response to him was:

I agree completely, Robin.  I love the cross valves.  Years ago,
I changed my cross valves to round when I was having trouble with
Durrell's leather leaking.  This was before I understood that most
leather today is too leaky to use on Duo Art.  I got worse problems and
changed them all back when I found good leather to replace his with.

The only time I had a set of cross valves leak is when I had bad
leather that leaked with the round valves as well.

D.L. Bullock    St. Louis
www.pianoworld.us