Thanks again for the help. Been listening to it nonstop for the last few days, making the music really enjoyable all over again.
I figured I should post some pictures and story of my build, as it's somewhat unique.
I built this all inside a very old microphone preamp enclosure from the 1940's from General Radio, made down the street from where I live now. I picked it up at a garage sale for $2, already missing some parts, then went looking for a good headphone amplifier design to build inside it, and found this one because I have about 8 6DJ8's in my tube collection.
Build went well. The challenge was fitting everything in the small case (about 7" x 5.5" x 2.75"). I was afraid there would be hum due to the proximity of all the transformers to one another, but in the finished amp it's barely noticeable, mostly only at power-up. Whew. While tightly packed, the whole thing is well grounded and isolated so I feel confident in it. And it's running great and stable.
Currently have it with Amperex Holland orange-globe 6DJ8's now and it sounds glorious. Thank you for the design and for the help, it was a ton of fun and I learned a lot in the process—my first end-to-end build of something new from the ground up, as opposed to a rebuild or restoration.
Full story in this sequence of pics:
https://imgur.com/gallery/xiOGB1A