I have a feeling that the ZDT would work wonders with the Moth Cicadas. Provided you can find a pair. However, at the LA meet in October, Craig said that he's looking to reintroduce the Cicada or a different driver. Hopefully, he can work his magic again.
The Cicada is similar to a Lowther, so you should consider those. Fostex makes a line of good sounding single drivers, as well. The Jordan JX92S is a fine speaker, as are the field coil offerings from Fertin and Supravox. If you want to buy new, Cain & Cain makes a few models with Fostex drivers. Klipsch will work, too.
A little off the beaten path, you might want to look for a pair of ESS AMT speakers. Unlike most ribbons, electrostats and planars, an AMT is easy to drive. I've had great success hooking a pair of ESS AMT-1 speakers to a 3W Moth Si2A3. Getting the planar sound from a SET is just amazing.
I'm sure I've left some terrific speakers off this list, but those are a good place to start. You will be able to find a number of efficient, excellent speakers for the ZDT.