I could be wrong, but AFAIK a D-tuner isn't designed to drop the pitch by much more than a whole step.
D flat is already a step and a half down - can they normally be set to go lower than that?
The single-stop Xtender drops a whole step. There is a double-stop model and it allows to drop in two whole-step increments so it can go to D, and then C, or even B depending on string gauge.