Also if you want DTS HD MA and Dolby True HD to be losslessly decoded, you will need to disable DD and DTS passthrough, as otherwise I think it will bitstream passthrough the lossy DD/DTS core/secondary streams rather than decode the lossless DTS HD MA or Dolby True HD losslessly to PCM 5.1/7.1.
You won't get Atmos until HD Audio bitstreaming/passthrough is supported on the Pi 4B - PCM lossless decode is only for Dolby True HD and DTS HD MA - not Dolby True HD with Atmos or DTS:x. (You may - possibly - get DD+ with Atmos - but that's not intentional and usually only used for secondary language audio streams on BD/UHD BD)
However if you want the best quality audio possible with a Pi 4B currently - disable passthrough, configure for 7.1 if you have 7.1 speakers (i.e. Front R,L, Centre, Side R, L and Rear R, L + Sub) or 5.1 if you have 5.1 speakers (note I'm ignoring 7.1.2. or 5.1.2 Atmos in this - if you have 7 speakers but 5.1.2 then configure for 5.1, if you have 9 speakers in 7.1.2 then configure for 7.1)
Interesting, thanks for your input. I had to try right away. I used a sample with Dolby HD and DTS MA in a highly non sciency test back and forth for about 30min between 5.1 7.1 and passthrough. My reciever was set to Neural:X and the same volume through out. My speaker setup is 5.1.4
7.1 seemed to give a better enveloping surround sound than 5.1.
7.1 was also slightly better than passthrough, sounded as it might have given slightly higher volume somehow...
Highly subjective and spontaneous.