Ok I get that. Unfortunately MPD does not procude any output by default on my system with the image I have received so posting the recommended settings would certainly help. But what I learn is that technically I do not need MPD because Spotify connect is a stand alone player. On the contrary, I cannot use these two players in parallel (which seems odd because that is what mixers are for, but maybe that is different on the Pi driver model).
But what about the Spotify Connect issue I am having? There is basically nothing to configure on the plugin page except what I have posted above. The error message cannot be a user driven issue, but it must be caused by the way the plugin is implemented. Did you test the plugin on the current max2play image with the JustBoom DAC?