I play with a guy who plays 12-string guitar so when I had my dulcimer built I went for volume with six strings (three double courses) which I usually tune AA-AA-DD'. The bass pair I made a high-low D like a 12-string guitar. It makes for a great-sounding instrument with low notes that don't give the usual "tinny" dulcimer sound. This tuning is very good for celtic music and/or when the guitar player is in DADGAD. I also had my builder incorporate one-and-a-half and a six-and-a-half frets so I can get much more variation in chording.