Legacy comms does make this (mostly) easier, but that would rule out the M6 rears without a bit of soldering...
If I decide to use the M6 I could use one of the 218 was the source and controller. Today, I turn on my LCR an set it to volume 82 because I calibrated the system with the JL Audio subs at this volume, but to control volume I use the Arcam. As I recall the 218 setup page allow me to set a start-up volume and source.
If you're using legacy comms cables after the master speaker, the comms connection to the master speaker input should be ONLY RJ45 pin1 to DIN pin 5 and RJ45 pin2 to DIN pin 2 and shield - no connection to DIN pin 1 at all. The Wiki diagram is OK for a slave speaker, but the commoning of 1 and 5 on the DIN connector will push master comms to the slave speaker if it's a master connection.
I will give it a try. Unfortunately I don't recall all the possibilities I tried in the beginning of this year, I remember that I could control the master speaker from the 218 SpeakerLink Master output, but it doesn't send the commands to the other speakers in the daisy chain.