Originally Posted by EricPode
I understand from the manual that it is possible to make the HD621 down-mix everything to stereo CD quality.
It isn't the HD621 which realises the downmix.
It gives the information to the sources to output stereo only.
The sources realise the downmix themselves.

But works fine if you disabled MHR (perhaps you will be limited to 48 kHz max).

861v6 + HD621 / Cabasse Baltic