Encountered a problem, then found the solution all at once, so I thought I'd share my experience:
Suddenly, while listening to music on my 707, the receiver unit would power down to standby mode, with blinking standby light. Not a good sign.
I tried again at lower volume, and found that the left-front channel was out. One of the speaker wires was loose, and it seemed that the vibration from the speakers made the wire rapidly connect/disconnect, causing the receiver to emergency power off. Re-secured the speaker wire, and no more power off!
Other details: I use banana plugs on the receiver ins, but just screw-downs on bare wire at the speaker end. Speakers make loud noise, loud noise = vibrations, so the screw-downs come loose, so the wires come loose. Something to keep an eye on