Welcome to my world.
Lay the amp back and remove the speakers. You might be able to remove just one and get it. PLEASE use masking tape labels, draw a chart, do SOMETHING so you'll know which wire goes where. Don;t assume you will remember. Page 9 of the service manual does show colors, but use labels. Free the wires from the speakers and reverb pan, and fish them up through the hole as you extract the chassis.
The mains wiring is simple - mains cord, fuse, power switch, transformer primary. Got 120VAC at the black wires for the power tranny? No? The find out why. If so... Power off, and pull the black wires off the board. Measure resistance between them. Open or got continuity? Open means the thermal fuse INSIDE the transformer has opened. Or much more rare the winding itself has opened.
COnsidering your symptom, you might only need a couple inches of access space to determine cause. SO se if that can happen before tearing into the speaker vault.