It is a good question. I actually did get confirmation from Dish, but it still may not be true! I think I'll disconnect the Toslink. Maybe it can't do both at the same time.

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I confirmed with Dish and several forums that the ViP722 can send DD5.1 over HDMI.

If that fails I'll do as Jack suggested and connect the Dish directly to the HD621 and see if that helps.