ok, so I've got MCR510 now from tuesday.. sound quality compared to PM6004 with rDac? No difference at all. Simple as that. Tested with wide range of music on the same pair of speakers - Q300. Headphone output? Again, no difference. 510 is really plasticy beyond imagination. How come Marantz released such low quality plastic thing in 60th anniversary? No idea.. but I would pay £100 more for more effort. It's this type of equipment one would like to hide on lower shelf and forget. It's possible as 510 can be operated via iDevice. Marantz remote app? Top 3 in the worst app I've seen. Luckily theres Airplay so Spotify's own app can be used with better results.
Everything else - like connectivity, set of features - you'll get from the specs but I can assure you the two amps/ dac's I have compared sounds the same.