Hi Whit,
From D,A,A simply raise the bass D to an E - giving E,A,A I use a reverse capo on some of my dulcimers which I just slide under the bass string next to the first fret. On other dulcimers, where I have a slightly lower gauge bass string my bass string will tune up the tone from D to E. E,A,A gives you all those mixolidian fiddle tunes (OJC, June Apple etc) and any ionian ones (Buffalo Gals, Cripple Creek) if you either have a 6+ or simply avoid using the 7th of the scale.
From D,A,d I again raise the bass D to an E (either re-tuning or with a reverse capo) giving E,A,d and can then play the A dorian tunes (Shady Grove, Elzic's Farewell, Cluck Old Hen, Cuckcoo, Frosty Morning etc). With a 6+ you can also work all the ionian and mixolidian tunes around this tuning aswell. So on 90% of modern standard dulcimers E,A,d will cover off everything the fiddlers play when they re-tune to A,E,A,E
I also use A,a,a and A,a,d depending on the sound I want.

too funny