A major cause of amplifier (or receiver) failure is shorted speaker wires. Even a single filament between the red and black terminals can cause severe damage. In certain cases, the damage is so extreme it cannot be repaired cost effectively. In many cases, especially with vintage gear, a bare wire connection with a high quality 16 guage wire will do fine.
If your equipment accepts banana plugs, bare metal connectors often used with large speaker wire should be avoided. Plastic insulated connectors, or rubber insulated should be used instead.
This short video illustrates a few important points.