I'm looking for additional information about bass dulcimers...size, internal bracing, etc. to compare with what I currently have.
Welcome back Jim
It is combination of the 2 although tuning is the truly unique part of it.
The best way to describe it is to check out my site and see the Youtubes and the pictures of them. They are generally "boat" shaped, deeper sides, but the unique thing that really sets them apart is that all four equidistant strings are tuned to the "D" above middle C or in some cases to "C". This puts them in a register all their own which lets them cut through in an ensemble. Because of the tuning you can play in D and G without retuning or C and F without retuning if you tuned to C. The "reverse" capo for the last two strings can also give you the ability to play in minor keys and some modal things without retuning. Hope this helps :)