My beta tester journey for Yamaha A8A continues. After discussing with the dealer to give it back, I reset the AVR and before disconnecting I said, let's make one last test since I can crank the volume up because I was alone. I restored the settings from a back-up and somehow the AVR started to sound right. I checked, music, movies, TV. The hissing is gone, the sibilance is normal, the center channel sounds right.
My guess is that the culprit is the connection with the web setup since I altered all the settings from the tablet and I could see that the AVR did not modified some of them. I do not know how to explain this.
Now the only big issue remains the popping sound. I discussed with the dealer and I asked them to contact Yamaha about the pop sound in the speakers and ask if it is software or hardware related. I could replicate the behavior at changing sources, when pausing and starting Atmos content, when I turn off the AVR.
For now, my beta tester journey continues.