Thank you all. I guess since most of the time I play in Double D tuning (at least with the folks I play Old-Time tunes with), I've worked on playing out of different keys without retuning, especially playing tunes in an Irish setting where two or three tunes in two or three different keys might be played together as a set. So far with what I've played with, keys that work well in aDADE tuning are G, Dm, Am, Em, Bm, and of course D. A works as well, though I haven't done much with it. In an Old-Time setting, it works well for switching between D and G tunes on the fly without retuning. In a way, I guess I've done with Double D tuning what Bluegrass banjo players have done with Open G.