Might be worth a quick email to Maton Guitars in Melbourne Australia. They used to make an awesome acoustic bass guitar which would rival a double bass for sound level. While the jumbo bass has probably disappeared from their catalogue, they may still provide or recommend strings.

As a double bass player I occasionally browse the DIY Build an Acoustic Bass pages. There's lots of them in all sorts of shapes and sizes and they tend to link each other's sites. The subject of string selection (or construction) often comes up. As I recall Dennis Havlena's excellent site even shows a technigue for winding your own. His site used to be at Mudcat Links - in any event "Havlena" is pretty easy to find via Google.

Regards, John