Strings
In December 2015, Salve Håkedal wrote to all hardanger d'amore players with some string advice:
Strings: I send the Hd'A away with hardanger fiddle strings, because it's essentially a hardanger fiddle. (The C is Dominant viola, light.) You may use different strings, but I encourage to use thin gauge. It's not built for heavy strings or even medium gauge strings.
Bridge: As the G-string in the hardanger fiddle string sets are thinner than both the C-string and the D-string, I give the crown of the bridge a little hump under it. This is so the G-string will be lifted up to the bowhair. If you use normal violin strings, the G will probably be thicker, so the hump on the bridge curve should be removed for optimal playability.
Sympathetic strings: If you attempt to fit normal hardanger fiddle sympathetic strings to the Hd'A, you will probably struggle to reach the two pegs closest to the head. I have made Knut Oppheimsbakken at Valdres Folkemuseum make longer sympathetic strings and will try to keep them in store for you. But you won't have to change them often.
Dan Trueman did extensive string tests when he got the very first hardanger d'amore. The strings he uses are:
C: Chorda silver viola 22.5
G: Eudoxa G 15.5
D: Eudoxa D 16.5
A: Eudoxa A 13.5
E: Gold Label E medium ball end