Body volume differences, wood differences, internal bracing differences, age of strings, many other factors will audibly affect the quality of sound between two dulcimers. Finish... not so much, IMHO.
The first three dulcimers I made for practice were exactly identical in materials and dimensions, but one was unprotected craftboard, one had just enough of a poly stain mix to coat the surface, and the third had a heavy few coats of poly forming a shell around the wood. These three were all made from craftboard so not exactly identical grain or anything, but the difference in the three was stark. The bare craftboard has by far the softest most mellow tone, and the one with a heavy poly coat is very twangy and bright, while the one with a modest amount of poly falls in between. The one with the most poly is loudest, and the one with exposed wood is the quietest, also. My assumption has been that because the poly is stiffer than the wood, the more i add, the more the tonal quality resembles a stiffer type of wood. I dont know how this would translate to harder types of wood. Maybe if it were a wood that is stiffer than the poly it would actually dampen the sound?