If all else fails, study the circuit diagrams in the Service Manual.

Focusing on the power supply, I probed around and confirmed that AC voltage was getting to only some of the places where it was needed. In particular, it was not getting to the inputs of the giant transformer with the orange "Sony ES" label.

Power was not passing through a power relay. The relay is activated by a chip, when the user pushes the power switch. Wait a minute, if the relay needs power to turn on the power supply, then what powers the relay?

Once I asked that question, I figured out that this Sony receiver has a small power supply with a small transformer, that provided standby power even when the unit is off. When the power button gets pressed, standby power activates the power relay, which sends power to the giant power transformer that runs the Sony receiver's powerful amplifier.

Knowing this, you can proceed to check the Standby power supply. It should be producing DC power any time the Sony receiver is plugged in (i.e. even before you push the Power button).