We recently experimented with some different strings on our Hurdy Gurdy and were amazed at how much these new strings improve the tone.
They are more expensive (the new strings cost $99.00 and the old set was $39) but we feel the improved tone makes it worth the price. You can purchase the new strings here.
The new set uses viola and cello strings as follows:
(2) Viola D strings (these would be tuned to the note D4)
(1) Cello C string (tuned to G2)
(1) Cello G string (tuned to D3)