Muted sound on left channel via toslink Ubuntu 19.10

This is a issue that has been irritating me for a few months now. I, a Linux-newbie, installed Ubuntu on my htpc early this year. It runs Ubuntu, no Windows.

My active wireless speakers (Devialet Phantoms) are connected via toslink (S/PDIF) from the pc to the speakers' hub (Devialet Dialog). When I play music via Wi-Fi (Tidal) everything works perfectly, but everything played via the HTPC has a muted left channel. The right channel plays at full volume, the left channel at ca 30% volume. Stereo, not 5.1.

At first, I suspected that this is a speaker/speaker firmware issue (there have been a few), but the manufacturer's support hasn't been able to provide any working solution. Now, I wonder if it might be ALSA-related, if there is a setting that I need to change. And ALSA seems a bit complicated, I fear. So I wonder if this problem is documented and if there's a solution available that a new Linux-user can make use of.

Added info:

I don't use any htpc special software, only VLC, Clementine and mpv. Pulseaudio volume control's configuration is set to digital stereo (IEC598) output. The output device is set to port: digital output (S/PDIF). But Ubuntu likes to reset this to its "default" HDMI/DisplayPort 2.

Is it possible to set Ubuntu to keep my settings?

Now, I have to change back from default HDMI to my preferences every time I start my system.

How do I set the volume for each channel in pulseaudio? There's only one volume control.

OS: Ubuntu 19.10
Материнская плата: Gigabyte Z390 I Aorus pro wifi

$ amixer -c 0
Simple mixer control 'Master',0
  Capabilities: pvolume pvolume-joined pswitch pswitch-joined
  Playback channels: Mono
  Limits: Playback 0 - 87
  Mono: Playback 60 [69%] [-20.25dB] [on]
Simple mixer control 'Headphone',0
  Capabilities: pvolume pswitch
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 87
  Mono:
  Front Left: Playback 87 [100%] [0.00dB] [on]
  Front Right: Playback 87 [100%] [0.00dB] [on]
Simple mixer control 'PCM',0
  Capabilities: pvolume
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 255
  Mono:
  Front Left: Playback 255 [100%] [0.00dB]
  Front Right: Playback 255 [100%] [0.00dB]
Simple mixer control 'Front',0
  Capabilities: pvolume pswitch
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 87
  Mono:
  Front Left: Playback 87 [100%] [0.00dB] [on]
  Front Right: Playback 87 [100%] [0.00dB] [on]
Simple mixer control 'Front Mic',0
  Capabilities: pvolume pswitch
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 31
  Mono:
  Front Left: Playback 0 [0%] [-34.50dB] [off]
  Front Right: Playback 0 [0%] [-34.50dB] [off]
Simple mixer control 'Front Mic Boost',0
  Capabilities: volume
  Playback channels: Front Left - Front Right
  Capture channels: Front Left - Front Right
  Limits: 0 - 3
  Front Left: 0 [0%] [0.00dB]
  Front Right: 0 [0%] [0.00dB]
Simple mixer control 'Surround',0
  Capabilities: pvolume pswitch
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 87
  Mono:
  Front Left: Playback 87 [100%] [0.00dB] [on]
  Front Right: Playback 87 [100%] [0.00dB] [on]

$ inxi -AG
Graphics:
  Device-1: Intel UHD Graphics 630 driver: i915 v: kernel 
  Display: x11 server: X.Org 1.20.5 driver: modesetting unloaded: fbdev,vesa 
  resolution: 1366x768~60Hz 
  OpenGL: renderer: Mesa DRI Intel UHD Graphics 630 (Coffeelake 3x8 GT2) 
  v: 4.5 Mesa 19.2.8 
Audio:
  Device-1: Intel Cannon Lake PCH cAVS driver: snd_hda_intel 
  Sound Server: ALSA v: k5.3.0-55-generic