H Tom - Most people do wash off their Dahlias tubers. I have such sandy soil, that I can just brush them off, but if you have clay it will take some water - and yes- then dry them well before storage. 55 degrees is warmer than optimal for storage. 40 degrees is recommended. If there is a closet on an outside wall, and you can put them on the floor, that would be cooler. How dry or humid the area is also effects how well they store. Peatmoss or vermiculite are the the most common mediums to store them in. It will wick away "sweat" from the surface, but help them retain enough moisture. Apparently at Swan island Dahlias, they just layer them with newspaper. I actually wrap each tuber in saran wrap, after dusting it with cinnamon as an anti fungal. I am afraid it takes some trial and error to find what works best for each of us.
They multiply a lot faster than iris don't they? Sweet potato sized is unusual, but I have had a few get that big. They big ones do tend to store better. Tiny ones are difficult to keep from drying out to much.
As for a tuber that never sprouted, but is still healthy - I am flummoxed. I'd save it over and try it again in the spring. Usually if they don't sprout - they rot. I have a couple that sent up a small stalk, but never got over 12" and didn't bud out, but still look decent, that I will give another try.
Good luck with them.