I've been using sources -> HD621 -> Radiance Mini-3D -> TV for the last few years.

(Recently, yes, I inserted one of the standalone Darbee boxes in between the Lumagen and the TV. I wouldn't say it made a life-changing difference, to my eyes at least, but I don't hate it either.)

But anyway - using the HD621 to switch sources conveniently as instructed by the 861, extract audio, and pass on the video to whatever does... video things, that seems to me like the intended use for an HD621. And it's been working fine for me in that setup.

Photographer, computer geek, listener, lad-about-town.
{ Lumin U2 Mini, ( {Apple TV, other HDMI gizmos} -> HD621) } -> 861v8 -> BAT VK-6200 -> {2x804S, HTM3S, 2xDS8S} -> ears.