I'd like to thank each of you for your great ideas, tips and advice. Rather than purchase another dulcimer, I decided to experiment with my 8-String Milford Blevins model. It has the longest VSL and widest fretboard among my dulcimers. I built a new nut and bridge to accommodate a 4-String DAD tuning setup, with 1 Drop D string included. Attached is an audio sample of what I've been wanting to do on Sal's Got Mud. I play each part once. Part A & B are played on the 'D' melody string, then part C is played on the middle 'A' string. Surprisingly, I don't miss the extra drop D and A strings. The wider fretboard allows me to comfortably pick the middle A without contacting the other strings.
The tonality and sustain is great. I did cut another sound port in the side of the body, which is matched to the opposite side. I originally determined that the top sound holes were not allowing enough of the sound to escape. These two sound ports really make a difference. Do you like the drop D drone? Does anyone else use a drop D string?