Thank you for the replies.
For my usage as a quarantined guitar teacher working via ZOOM I am just trying to preserve the stereo Kemper sounds in while leaving the Apollo mic pre's open.
I was originally using a cheap behringer mixer in between the Kemper and Apollo to get Kemper stereo out, a vocal mic (also picking up acoustic guitar) and my iPad running spotify and the Amazing Slow Downer app for transcribing/ content video production. The extra gain stage was making things a little complicated and changing the Kemper sound a bit for the worse.
Edward, There are lots of spdif to optical converters out there but I was looking for one that has been found to function optimally in this situation to save time. I had a Kemper support answer recommending that this may be a frustrating journey as no add on unit can be verified by them so I gave up on it for now. I took the suggestion to go mono master out to Apollo for now. This leaves the second pre open for my vocal mic and now I just have to free up a usb port in my iMac for my iPad to get the Slow Downer audio in.
I would like to solve it another way by adding channels to the Apollo or getting the Optical in working eventually so that I can go between teaching and recording with stereo sound without changing my routing.
Scratch, that is some next level usage! I will file that away in case I feel that something in the Kemper fx chain may beat something in the logic plug in realm. I'm still on the surface level usage with this thing but happy to learn.
Honestly I doubt Zoom is stereo. Just do it mono XLR in the input 1 and then put a microphone in input 2. Select Universal Audio as your input in Zoom. Done.