I've found a situation where the output is muted even when you adjust the volume level. It is likely to only be possible using RS-232 commands.

Steps to reproduce:

Decrease the volume all the way down to -76db - you can use IR or RS-232 for this.

This will shut off the 990's pre-outs.

Issue the RS-232 Low-Volume preset command to set the 990 to exactly -40db

Note that output is still shut off - there's no audio.

Increase or decrease the volume (only tried RS-232 for this) and you'll notice output is still silent.

Mute and UN-mute to bring output back.

Who does this knid of stuff for fun?

I found it while setting up my home theater controller with a fail-safe volume command. A button that can be held to restore the volume to a safe listening level, regardless of the situation. It just turns out that if the situation is that someone has turned the volume all the way down, it doesn't work.

Frankly I'm surprised it wasn't found in testing.

My kind of people...