The HD621 User-Guide states:

"So, for example, if you have a 2-channel system and limit the PCM output here to 2-channel, you only need to connect the “1/2” SmartLink output from the HD621 to the system."

So we now 1/2 is for Front Left / Front Right. However, the relationship of the others is implied and not stated (M should update the manual to clarify this!)

This is how I have it setup and it works.

HD621 (3 * SPDIF) --> 861 MD1-123 (or MD2-456)

DL/R 1/2 ------> D1 (D4)
DC/LFE 3/4 ---> D2 (D5)
DSurr 5/6 -----> D3 (D6)

Plug them all in and then play a channel ident test disk to confirm all is well.

Regards,
Carl.

861v6+ID40, HD621, 8000.2, 7200HC, 7200.2, SKYQ Silver, PS3, PDP-LX6090, QNAP TS870-Pro (8*6TB), Roon, iPadAir2.
[In Reserve] C15, MD600 (2*3TB), TwinStore RM (2*2TB)