Abel hammers are my choice for replacement.  If you prefer brightness,
then install Renner.  I have not encountered NuTone hammers for many
years, now preferring Abels.  They need the least amount of voicing.
Are you changing the shanks as well?

The tails can be shaped to any contour and the hammers bored to any
length, so I am not sure what your tech is talking about unless he
installs them pre-drilled as received.  I would search out a tech who
is experienced in installing custom hammers.  The string heights can
be different from section to section and the hammer lengths need to
mirror this; "off-the-shelf" sets are not always the best choice.

You mention weight; get a tech who is experienced with weighing
hammers.  Many techs now are graduating the weight to within .1 gr
of each other.  When plotted on a chart a smooth line will read from
about 9 gr to 3 gr.  Shank strike weight (2 gr to 1.5 gr) needs to be
figured in as well, so that once the hammers are glued onto the shanks,
a smooth Strike Weight curve is the result (11 gr to 5 gr).

This is an average, your desired weight could be different.  But the
action geometry and Strike Weight Ratio needs to be addressed because
excessively heavy actions can result from improper shank selection
(knuckle placement on the shank) and strike weight selection.  Any
action can be improved by someone experienced with their function.
Check out:  http://www.stanwoodpiano.com/first.htm

Regards,

Jon Page,   piano technician
Harwich Port, Cape Cod, Mass.
mailto:jonpage@mediaone.net