What dlna servers have you set up on the client mc and the debian mc and have you associated them with your renderers?
All DLNA renderers appear in both the client and the server, and historically they have all played without issue once I had set them up properly. I'm not using any ZoneSync or anything like that and each renderer has its own dedicated zone.
I'm assuming that given they can all play flac files streamed from my NAS, then the FLAC stream from RP should play as well. I'm a little reticent to play around with server settings etc as they all work really well.
@Jim my CXR doesn't convert files at all, set to original, and the JBL only converts files that have a higher rate than 88khz, I wouldn't think these would cause any problems with the RP stream given it is 16/44 flac.
I'll update both MC systems to those suggested later tonight and see how that goes.