Pardon me for resurrecting this thread.
I have a lute-guitar, a 100-year-old one that I just had a little restoration work done on. Technically it belongs to my husband, whose grandmother played it (and zither) in Germany as part of the Wandervogel movement. It has a lovely plunky sound. The scalloped frets are not very forgiving.
In this thread, the subject of strings came up. I'm still trying to figure out what strings will sound best. I have it strung with Aquila romantic-guitar strings, tuned below A=440. I think the trebles sound okay, but the basses get an intermittent twangy, zingy sound that they would not on a guitar. I don't fully understand the physics of this bowlback instrument, but today I had the soundboard turned toward my face and plucked a treble string, and the intensity of sound made my tooth hurt (!) even though the instrument is not overall terribly loud. There's nothing wrong with my tooth afaik.
I might try gut strings on the trebles rather than Nylgut. But I have no clue about the basses. I noticed that a certain European seller carries a set of lute-guitar strings, but I see no specs so I don't know how they differ from other strings. I could just order them and find out what's in the package, I suppose.
I also don't know if all these German lute-guitars would have been built with the same strings in mind. I've seen some described as even older than this one. I've also found examples that are dated up through the 1940s and 1950s. And then there are the Roosebecks made today.
Would love to hear any further string suggestions, if you have them, based on the concerns I'm describing.