Very Weird Failure Mode for Denon AVR4802

We are using a 20-year old AVR4802 for 5.1 sound on a secondary TV setup with a U-verse STB. The STB was set up for stereo output (for reasons that are irrelevant to this discussion) through a TOSLINK cable to the AVR, with HDMI going directly to the TV. The setup had worked perfectly the previous night.

I was in an adjacent room yesterday, and my wife turned on the system using the U-verse remote. I heard the most God-awful sound (like a car horn with two tones at once) when the amp turned on - it must have been 140 dB. I thought the house was on fire or the North Koreans had done something stupid. Luckily, my wife immediately turned everything off. After changing my pants, I went in to investigate further.

I turned the system on again, and it did the very same thing again. Now being in the room, I could tell that the sound was only coming from the surround speakers in the ceiling, and the volume control on the AVR could not reduce the volume at all. (And remember, the STB was outputting stereo only at the time.) So, apparently this sound was being generated in the AVR's surround processor or preamp somewhere and completely bypassing the volume control. Scary implications for speakers! Next, I disconnected the surround speakers and turned on the system again. You could hear the "horn" sound coming from the front left and right speakers, but just barely.

Obviously, this receiver is toast after a long and productive life, but I just wanted to see if anyone here had ever seen/heard anything like this?