Jack,

You have MCH sources and a 2CH output. Your question was whether the source devices or the processor do the mix down to 2CH.

With HDMI the target device tells the source what it is capable of accepting on its inputs. If you set the HD621 to tell the Oppo that it can acccept 5.1 then 6 channels of PCM will be delivered to the HD621. The HD621 will then output this over the 3 x SPDIF connection to the G68 which will then do the 2CH mixdown.

If you set the HD621 to tell the Oppo that it can acccept only 2.0 then 2 channels of PCM will be delivered to the HD621. The Oppo will have done the downmix. The HD621 will then output this over the first of the 3 x SPDIF connections to the G68 which will then output this as L/R.

Anyway, I do not have any 5.1PCM DVDs so I can't comment on the sound quality.

Malcolm