Still no sound from rear speakers in home theater mode

Hello, No sound comes from rear speakers in home theater mode. My setup: Soundbar+ - Connected to TV via eARC HDMI port - Connected to the network via Ethernet Amp - Connected to the network via Ethernet Sub+ - Connected to the network via Ethernet Apple TV 4K - Connected to TV via HDMI - Connected to the network via Ethernet - Settings: - Audio Output: TV Speakers (but also tried Airplay2: Temporary Audio Output - BlueSound Theater group) - Change format: Off (but also tried On with Format set to Dolby Surround 5.1) - Immersive Audio Dolby Atmos: On X Audio mode: Auto (but also tried 16 bit) Sony TV KD-75X80K - Settings: - Digital audio out: Auto 1 (but also tried Auto 2 and PCM) - Dolby Digital Plus output: Dolby Digital Plus (but also tried Dolby Digital) - Pass through mode: Auto (but also tried Off) Soundbar+ / Sub+ / Amp Home Theater group - Selected Soundbar+ as connected to TV - Adjusted relative sound levels (sound comes out of front and rear speaker with the corresponding test) - Audio settings: - Surround Upmixer: On - Virtualizer: On (but also tried off) - Volume Leveler: On (but also tried off) - Dolby Audio (Dolby Surround) logo appears in BlueOS app in Now Playing All the devices have the latest SW version installed. The Amp and rear speakers work fine when not in Theater group, as soon as I add them to the Theater group no sound comes out any more despite the Dolby Audio (Dolby Surround) logo appearing in the BlueOS app. The same Apple TV and content on a traditional AV works fine (sound comes from all speakers). I have tried many settings as listed above. Please help. Thanks in advance, Fernando