Player>Zone>Zoneswitch
Move the listed zones up or down in the order you want MC to look for the zone to play in. The top zone will be your default as if it meets the criteria it will play there. If it doesn't it will go to the next zone listed and so on.
I didn't remember this feature about Zone Switches. I was going to suggest two Zone Switches as well. My system is set up in a similar way. When I play music, I always want MC to look for my DAC and play in stereo. When I play movies and TV shows, I want MC to look for my pre-pro and play in the file's native number of channels.
Zone Switch - 2channel: is chosen when [Media type] = audio, it looks for my DAC as the Audio Device, and it stops playback in Multichannel (my other Zone).
Zone Switch - Multichannel: is chosen when [Media type] = Movie or TV show, it looks for my pre-pro and stops playback in 2channel.
I just have to remember to switch inputs on my pre-pro, which is very old school I know.