I took my previous analog output test and replaced AES3 DB25 with ADAT. Success! The audio comes out of the speaker after making its way out and back in via the optical out/in. For anyone in this thread looking to chain multiple FX chains together, this is a way to chain two, although you need a physical cable and it blocks you from using the digital outs for other purposes.

Unfortunately, switching to ADAT also breaks my intended monitoring. If I connect the optical back into my amp, it doesn’t play any sound. It seems to only be able to interpret S/PDIF (Optical) or the first two channels of AES3 DB25. So it seems I’m still stuck unless I can find a way to take an ADAT optical signal and convert it into S/PDIF for my amp, or maybe do something similar with an AES3 splitter + AES3 to S/PDIF converter.

I did run into something strange here though. I switch to ADAT and the amp stops working, if I switch back to AES3 DB25 (by loading the session again and discarding changes) it still doesn’t work. To get it to work I either need to reboot the H9000 or switch to S/PDIF (works) and then back to AES3 DB25 (still works). I’m confused how AES3 DB25 works in the first place actually since I would assume that would only be for the AES3 and DB25 connectors, not the optical.