edit: Sorted this out. Removed chassis ground (B
in above schematic).
This isolates the secondary side of the audio transformer from the chassis ground on the primary side. Thereby isolating it from the 100W tube amp's ground potential - thereby eliminating the 60-cycle hum.
Able to run the amp on 8 for twenty minutes with heat sink and power resistors merely getting warm to the touch, not hot.
Very transparent! Quite amazing, really. It basically sounds like the amp would
if I were standing outside my house and running the amp on master vol 8 - only at bedroom levels.
Tone is there. Saturated, warm, crunchy sustain, organic feedback at low volume.