After swapping out the tubes with better ones (Penta Labs for 12AT7s and Tung-Sol for the 6sn7s, not pictured below) I was able to wipe every single bit of hum. There is still an issue when the volume control is in the 11:00 to 3:00 position where hum comes in, but for now I am just leaving volume at 100% and adjusting from the source. But I now consider this a successful build! The sound is a big leap up from the SRD-7 I was using.
Thanks to everyone in the community who helped on this.
James Lin for the design, the great article on AudioXpress, and overall thought leadership
Kevin Gilmore for the board layout and guidance
gepardcv for providing KGBH PSU boards and being so patient as I resolved the exploding zener diode issue! I owe you, man!
mwl168 for doing the group buy and for providing endless guidance.
congo5 for doing the early prototype build and showing just how good this design is.
jose for doing one of the early builds and giving me guidance
and most of all, to my son who came up with this project and did most of the soldering after my exploding zener experience!