I've worked in the area of piano repair and rebuilding for
approximately 30 years and this is my experience with regards to
putting twist in wound strings.  In old strings that sound dull
or 'tubby', two things can be done to improve the sound.  These
involve making the string more flexible and adding to the twist.

Doing one string at a time, let down the tension so that it can be
unhooked from the hitchpin, making sure to notice how many turns it
'unwinds' when you pull it off the hitchpin.  The following works
better with steel-wound strings than copper-wound strings, but will
improve both.  Form a loop in the string, passing the end once through
the loop so it's like an overhand knot, leaving the loop about 3" in
diameter, or larger for the thicker strings.

Insert a hard cylindrical object (a glass beverage bottle works very
well) and run the loop from end to end of the string several times.
This loosens up the corrosion between the windings, making the string
more flexible.  Return the string to its hitchpin, adding one more
half-turn to the amount of twist it had before, and pull close to
proper pitch, then go on to the next one.  If the string had no twist
at all to it originally, put one full-turn on it before rehitching it.

With regard to new strings, I've rarely had ones "new, out of the box"
that buzzed when installed properly.  The makers of strings I've used
were either Schaff or Mapes.  The only strings that ever gave me trouble
were made by the now-defunct American Piano Supply.  When installing
new strings, I put one to one-and-a-half turns on the strings that
are in 2- or 3-string unisons, and a half-turn to full-turn on the
single-string notes.  Make sure the twist is in the same direction as
the winding, so it tightens the windings rather than loosening them.

One other thing is that if you use a string stretcher roller to
pre-stretch the plain wire strings, _never_ use that tool on the wound
strings.  Doing so will guarantee them buzzing like a hive of bees!

John Runge