One thing you should be aware of if you put the HD621 after the Radiance is that you lose the ability to add lipsync on a per-source basis... The HD621 would only see the one source - the Radiance.

In my system I use the HD621 as the switch and apply per-source corrections and then use the 861 to add further corrections for the Radiance's latency (which changes dependent on input/output resolutions/frame rates) and codec choices.

Just another thing to think about...

My Music : My Cinema