That's it! I had the cable plugged into the SpeakerLink Input connector not the output connector (the 818 is in a rotating rack but with somewhat poor visibility to the back).

I haven't tried it yet, as we have company over for dinner right now (getting harsh looks from my wife for being on the computer with company over), but I'm sure that's the problem. Will report back later.

Can someone answer the question about whether the Meridian setup program can read the current configuration or just write new configurations?

Thanks everyone!

Main System: MD600 & Sonos Connect -> 818v2 -> DSP5200SE+DSW SUB
Alternate Main System: Sonos Connect AMP -> Dynaudio Contour 1.3 SE speakers + Sonos SUB
Kitchen: Sonos Play:1
Bathroom: Sonos Play:1
Bedroom: Sonos Play:3