Recently been playing with my DSP7200s and HD621 and can confirm the configuration should work.

In my case (for various reasons) I removed an 818 and replaced it with a G68, then replaced that with the HD621 direct.

Now I do not remember changing the Type on the DSPs so they probably remain at Type 5 or Type 6, i.e. telling the speaker to output source changes over comms. This configuration is covered on page 32 of the DSP speaker manual.

So perhaps your problem is actually in the DSPs and not the HD621?

Good luck.