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Do the tubes - all of them - in the amplifier light up when powered On?
(If not, the power for the cathode heater is missing. That will indicate to check connections for that power, and components from there.)
Examine (after cleaning dust off, as needed, but quite thoroughly and with the power Off and unplugged for sure) the circuitry. Are there any brown components, brown spots on the boards, leakage from capacitors, etc.?
(Note: capacitors that leak from that era of equipment, esp. any tarry dark sludge, should be treated with extreme caution and care. Many capacitors in those days had PCB electrolyte in them, which is toxic and carcinogenic.)
Check generally for broken solder joints, loose or decayed or frayed wires / connections, etc.
When it doubt, swap speakers also. At least once. Leaving wires in place, but re-doing at the speaker end (if possible) just to be quite sure.
Let me know what you find, and I will advise you further toward a solution for this problem.