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.