Ah, interesting. So it seems that for streaming from an Apple TV 4K, it will output 5.1 LPCM at 48kHz which an optical connection should be able to handle. So no advantage using an HD621 over optical for streaming from an Apple TV 4K?

#1: 861v4 | HD621| 218 | DSP8000SEUP | DSP7200.2HC | DSP5200 | SW1600
#2: AC200 | DSP3200 | DSW