First my disclaimer: I have never built any Bowed Psalteries, I am just an avid BP player and sometimes a luthier groupie! I have however adjusted the pins, strung, and restrung hundreds of Bowed Psalteries new and old. I see older BP's that had wound strings put on them. Often the wound on wire has worn to breaking and unraveling where it goes over the top of the rail pin. If you look at an AutoHarp you will see they have the wrapping stop before the core sting goes over the bridges to avoid this problem. The Violin/Viol families use FLAT wound string. Regular wound string made for plucked instruments end up sounding more raspy or "wiry" and often when bowed actually resonate one octave higher. Now a days very expensive flat wound strings are available for Guitar and Mandolin you might give a try.