I figured it out after mulling over things in my head last night. The ADAT outputs on the MR816x are turned down by default. The only way to turn them up is either to use the MR Software (outside of Cubase, without Cubase running) or to use the Audio Hardware Mixer in Cubase. In order to use the Audio Hardware Mixer, the outputs must be assigned to something.
Seems silly that the default is to have the channels turned down. Of course, the MR816x saves the last used configuration so now that I’ve turned them up, they should stay that way.