Maybe one small step at a time?
You say the tube neck doesn't glow on a working unit, I'll take your word for that, they are sometimes very very dim. In most cases if you put all the lights out and look, you can see at least a dim glow which will give you some confidence that the tube heater is being powered. Unusually, the tube heater supply in this is not derived from an output on the Line Output Transformer, instead the tube heater seems to be powered straight from the 12VDC output of the main regulator.
You have the power LED on, and you have sound so that suggests you have 12V coming out of your power supply, but you should still check that anyway just to make sure the voltage is correct, because a fault in the 12V regulator could potentially pass the unregulated input voltage straight through onto the 12v rail. You should be able to find +12VDC and 0V on the inner two and outer two pins of connector P901.
If you have that, check the output of the +5V regulator as well, IC301.
Check the voltage from both ends of R701 (120R) down to 0V - should be +12V on one end and not much less than +12V on the other end. This resistor supplies the line output oscillator, without which there will be no drive to the Line Output stage.
Check at the junction of D707 / C716. Again, there should be +12V there, that's the supply point for the line output transistor and transformer.
Do you have an oscilloscope?