Beardy,
I'm not an expert on hum, but here is my feedback:
I'm not sure you need to twist filament heating wires if your filament heating is done with DC. They will not emit 50 (or 60) Hz. Still you may want to twist wires carrying audio signal, e.g. from input to pot and from pot to amp board so they do not catch whatever parasitic waves flying around.
Myself I have used a "star" config for ground. See on the pic below, the red stick point to a small ground connector where all the ground wires (brown) come. One from PSU board, one from tube board, one from the jack output (bottom right) and two to each pin 8 of my OPTs. Plus, the two small blue capacitors from B+ with a wire extension isolated in black.
You may note the respective orientation of the PSU transformer and the OPTs, so that they do not induce fields from one to the other. The other important point is how you connect input earth to audio ground. I used the exact setup from the original scheme (120R + 0.1 uF-X2).
Another potential source of hum is a ground loop. Try to use a floating source for your tests, e.g. a battery-powered smartphone with only an audio cable to the amp and not connected to a power source. Or short-circuit the input, the output should be silent.
The other things you see are not relevant: a led and its resistor at top-right.
Attachment:
IMG_20180123_211848.jpg
The way it is now, it's dead silent without music; and extremely good sounding with music.
I'm not very proud of the box and the cabling, I used the wrong wood (beech) and it's twisting a lot. I will rebox the thing in a metal-case over the week-end.