Kinda like coffee me thinks... Sometimes bold with hints of chocolate or cinnamon....Or bland without any flavor.. All subject to the ear of the listener. Perhaps sometimes so many flavors it just tastes "off"...
I don't think I've heard a dulcimer yet, that didn't sound sweet.....
So the term "overtone" as it relates to dulcimers, just means perhaps you're thinking too much......Or you're building one out of an old toaster and a bundt pan .....