It's been a while since I've done the whole bitstream vs. PCM issue as I've set every other AV device to bitstream, but the ATV4 should be set to PCM as it defaults everything to DD even when it's just stereo as that's either a bug just a horrible way of forcing everything into a DD format.
Really, the volume difference is just how your receiver sets the volume levels. DD is set at a higher volume level than PCM, there's absolutely 100% nothing wrong with your ATV or your receiver, that's how PCM is. Hence the noted difference between Stereo/PCM/DD. I just set the reference volume on my ATV port higher than normal and it's the same as if I used DD.