>So what if you want to apply different DSPs to the difference sources?

George is correct. The sources connected to the HD621 are mapped as phantom sources on the G68ADV. Selecting a (phantom) source of the G68ADV selects the physical source on the HD621, applies the DSP settings and changes the physical input on the G68ADV to MD1 or MD2 as the case may be.

I was thrown off track by the error message and lack of documentation. The need to set the main audio input on the G68ADV should be documented or better yet done automagically.