That block diagram only shows the analog part, and I did look and it and concluded that if you use analog input, in direct mode, the zone2 pre out can be expected to have SINAD in the high 90s, even 100 because the PCM5100 will not be in the path.Hi @peng, Thanks for taking a look.
Referring to the DRA-800H block diagram supplied by @bloodshoteyed.
The AKM4458 analog out L & R goes to the InA/B6 of the 7in/3out matrix switch. The Zone2 would correspond to the OUTA3 and OUTB3 for L and R respectively on the output side of the same matrix switch. For there to be a connection, the user would have to somehow set a connection between InA/B6 and OUT3. My hypothesis is that this connection is set with "All Zone Stereo" but I do not know for sure that this is how to establish the connection. In any case, "All Zone Stereo" means that Zone 2 plays the same source as Main Zone.
I looked again and saw no evidence of the main zone DAC, the AK4458 would be used for Zone2. Take a look of page 18 of the SM and you will see 6 channel outputs of the 8 channel AK4458 are used for the left, right and subwoofer channels, it took 6 instead of 3 due to the differential connections, and the remaining 2 channels (if single ended are used) are not connected. I supposed that could have been used for the Zone2 pre out, but they are not used.
The other way for Zone 2 to play the same source as Main Zone is to use the "Zone 2 SOURCE" button on the front panel (button second from left). This button allows cycling though the various sources such as CD, blu-ray, etc. But if you select "Source" this causes Zone 2 to play the same music as Main Zone. Based on the service manual for AVR-S950H/X2600H (similar to DRA-800H with same PLD, DAC stack etc), when set in this way, digital signal goes through the "Downmix DAC PCM5100A" and the resulting analog is routed to Zone 2 via a 7in/3out switch. It is possible that this was the setting for the original measurement which no one was excited about.
Yes, that part seems obvious.