I'm working on it. Or, rather, gathering parts so I can build it after finishing off a few other projects.
I picked up book of projects from Glass Audio and there's a SET 45 based amp for speakers. Looking it over, I realized that you could reduce it to just one 45 pair for output and substitute in a pair of Sowter output transformers for headphones. Even better, the output transformers have multiple taps for different impedances.
So far, I have the tubes, sockets, some resistors and caps, and the output transformers. I plan to order the interstage transformers from Electra-Print. The power and filament transformers will be Hammond, as will the choke. I was planning to use a stepped attenuator on it, but I've gotten curious about some of the transformer attenuators out there... I might just go all out on this one. I'll also put in a switch and wire it so you can switch the output transformer taps to match the headphones.
Casing will be another big project. I'll make another by hand entirely out of aluminum. I like to pick up offcuts of aircraft aluminum on eBay - maybe I'll look for a nice 1/2" plate for the base, then build on that.
That should just about round out the wish list. I still want a 300B based headphone amp, but I haven't found that many DIY plans for one. Still, a 45 amp and a 2A3 amp are a pretty good representation of DHT tubes.