this unit?
https://schroederamplification.com/products/bryston-sp3-surround-preamplifier
Looking at the rear jacks, there are two tape loops, one analog and one digital. It has to be selectable, and other settings are involved to get to 2 channel, etc.
here's your manual, perhaps someone here understands these modern devices, not me.
http://support.bryston.com/downloads/sp3/documents/300024%5BSP3%5D.pdf