Some things to try:
1. Set MC's DLNA functionality as follows (can't remember exactly where in the options this is, probably Media Network):
DLNA Server: On
DLNA Controller: On
DLNA Renderer: Off
2. In Configure DLNA Server settings, there are two items DLNA and DLNA Extra. Try playing with different combinations of these two, on and off.
3. Also try turning audio transcoding/conversion on and off.
4. Can you use Windows 7/8 to Play To the Marantz? This will work by DLNA too.
5. Is there any setting on the Marantz that enables network control?
Rest assured, DLNA functionality does work in MC, but getting particular renderers to play ball is sometimes hit and miss! There's not a great deal of configuration that you can tweak, other than the above, so if it still won't work MC staff might ask you to provide a Wireshark dump of the DLNA traffic to see what error is occurring.