I do get this error in the logs for the sub:
Logger: homeassistant.helpers.service
Source: helpers/service.py:130
First occurred: 8:00:44 AM (1 occurrences)
Last logged: 8:00:44 AM
Unable to find referenced entities number.sonos_system_sub_gain or it is/they are currently not available
Does this error happen when you’re trying to call an automation or script? If you look at the parent device (Settings → Devices & Services → Sonos → Devices → parent device), do you still see the Sub Gain entity and is it available?
It only happens when I am restarting my server - the sub gain entity is there so I always wonder why it says it only on boot up of my server - never when its running
The warning is about a service call. Do you have any automations that trigger at/near startup which would attempt to adjust the number entity?