You are not reading the schematic correctly, it is fine. The LED circuits are not running off one side of the heater voltage, the DC supply the LEDs use is also used to elevate the AC heater supply. And that is why there is a e-cap at the hum balance control instead of a ground. If there was no DC on the heaters before, that was probably why it was humming in the first place. F3, CR6, C8 form a small + supply, I forget how much, probaby 20-25v or so. Then R10 connets that DC offset supply to the heater AC.