Carl, do you happen to have the formula for the vibration frequency
of a bass wound string?

As far as a supply of hexagonal wire, you might try one of the best
regarded string suppliers for piano techs: the Mapes String Company
in Elizabethton, TN, 423-543-3195.  As of several months ago, they
were not using e-mail.

Carolyn Macneal
Southwest Virginia