I haven't got a chance to load the 3.16 firmware yet as I was missing the USB to DB9M cable so will have to get one when I got a chance and I will give it a test and report back. I guess there shouldn't be any sonic difference between SL input or spidf?

I have used an unused source for me is "Disc" for the 218. So, when the 861 is in control the Disc button will select the correct 218 but you can see the 861 is a bit confused. If I turn the 861 off first the 218 will take over control and the 861 remains off. Interestingly, my rear speakers still comes on whilst in control of the 218.

Lounge: 861v8 with ID41, 218, UHD722, Sonos Connect Wyred4Sound, AppleTV, Oppo 105, DSP8000SE, DSP5200SE
Streaming: Roon/Spotify
Currently not in use: DSP5200 x 2 pairs, DSP5200HC, DSP3200, 218 x 2, MS600, DSW x 2, MC200, Sonos Connect Wyred4Sound