Harman Kardon Citation 17 Pre-Amp One Channel Intermittent
I've had bad contacts on relays before. Both audio equipment and a programmable power supply. To fix the power supply, I wrote a program to open/close the relay every 10s overnight and the problem went away.
I'd measure the DC at the speaker terminals before and during a fault first.
I'd expect something initially and probably zero if your loosing contact.
Then do the same, but before the relay.
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