If the KEF LSX II speakers are connected to your Linux (Mint) PC via Digital Output (USB audio) instead of WiFi and the audio volume is not loud enough, it might be, that the gain of the internal sound card of the LSX II is set too low:
To fix this, open alsamixer in the terminal:
Then press F6 to select the sound card of the LSX II and increase the PCM volume to 100% with the arrow up key:
Then save the settings permanently with:
s sudo alscactl store
Also check that the volume in the KEF Connect app under Remote is also set to 100%:
Now, you can regulate the volume in the Linux Mint Sound settings…
…or directly on the keyboard with the media keys. If your keyboard only has volume -/+ and play/pause buttons such as the Microsoft Natural Ergonomic Keyboard, but no previous/next track buttons, create two new keyboard shortcuts in the Linux Mint Keyboard settings:
This way, you can completely control the Wireless Hifi Speakers KEF LSX II with the keyboard (when playing music from Spotify).
Linux rocks !!!
Undocumented: You can access your speakers directly in the browser with the speaker’s IP address, e.g.: https://192.168.1.72/index.fcgi