Control Room - Monitor Source Input problem

I run group outputs to a Dangerous 2-Bus, through outboard and then do AD through a Hedd 192, returning to Nuendo via AES (Input channel named Hardware Rtn). I normally monitor this directly from the HEDD, but am wanting to use my Lavry DA as a monitor controller to handle multiple digital sources.

As far as I can understand, I should be able to do the following in Nuendo:
Setup input channel from AES (Hardware Rtn)
Add Output Channel via SPDIF, assign as Main Mix
In the Studio Tab - SPDIF Out are main speakers. Add monitor source > Inputs > Hardware Rtn

On the Lavry, set input to SPDIF

On the CR mixer, there are now two monitor sources, SPDIF Out and Hardware Rtn, and the tab for multiple sources. I deselect that tab and only select Hardware Rtn. Push play and can see the levels on the CR meter, which are now showing the levels from the Hardware Rtn - which correlate to both the HEDD meter and the Input Channel meter. But have zero output.

If I disable CR, and preview a file in Media Bay, it plays through the SPDIF, as expected. interface also shows SPDIF output. Now, when I re-enable CR, that particular file still plays through SPDIF (interface still shows SPDIF output). If I then preview another file, we go back to silence, though the CR Main levels continue to show signal and the interface shows no SPDIF output. Is this a bug or am I doing something wrong?

It almost looks as if you connected your interface’s spdif to Nuendo’s mixer under the output tab, instead of doing that in the “studio” tab. When using control room you’ll set to your destination there, not using “outputs”. You can almost think of the mixer as one software and control room as another. The input/output tabs belong to the mixer, and the studio tab control room. So control room allows you to “tap” the signals on the mixer, which means that you don’t want to select your monitor output in the outputs tab (mixer) but instead the studio tab.

The way I have it set up is I have a bunch of channels in the “output” tab, and none of them actually go to a physical output. Instead, I treat them as the “don’t touch” last-stage in my signal chain. All my stems and mixes end up on these outputs, and they don’t go anywhere. Then I use Control Room to monitor those outputs, and in the studio tab I select as my destination my surround speakers, my Grace interface for headphones or ‘crap’ speakers, or just a stereo set of speakers. So those destinations are all set in the studio tab, not in the output tab.

So, maybe you can post a picture of your routing, because it sounds as if something is off.