But Bluetooth is still not optimal via this system. When I use Bluetooth headphones from the same place with my mobile phone, I can almost listen to music in the whole flat. With the system, the reception barely comes out of the living room.
It's probably because the system has been around for a few years and in this case the problem can't be solved with software updates because it's the hardware of the Bluetooth chip.

I have found out: Via the Musicast app under Sound settings.
Important with headphones: Don't touch the volume settings on the remote control! Otherwise the neighbours will soon be knocking on the door. Because then the speakers will turn on again. So only adjust the volume on the headphones.