The Marantz video processing is preferred (to me) over DTV, and in-fact, the TV processing may be even better, so Native ON is best (OFF means DTV does scaling to what you set it to and I prefer Marantz or my projector).
The DTV video settings often switch back on their own to just 720p with the C61k (it does this on mine all the time).
This is a very well known issue that FW supposedly corrected, but did not.
One needs to go into the video settings and check that all boxes that the display accepts are checked. With the new OS, this is a great pain to get to, drives me insane.
This is the first thing to check whenever this happens, just to make sure it is on.
Also, just turning the OS off and on recycles these settings.
To the DTV, the display is both the AVR and display, so the Marantz input must not be restricted to a particular video format, unless you want it to be that way.
Theater: Lyngdorf MP-50 7.3.4, Sony VW675ES, Revel Ultima Salon2/Voice2/Gem2(4)/Paradigm Be Atmos (4), Mark Levinson amps (53 L/R, 532 Center biamped, 531H side/surrounds), Atmos amp: Sherbourn, Oppo 203, Roku Ultra Premier+, DTV 4k, Velodyne 1812 Signature (LFE), Velodyne DD-15.
FR: Marantz 8802A (and/or 7703), Sony XBR 4k TV, Meridian A350 LCR onwall, Dali in-wall surrounds, Atmos Martin Logan in-ceiling, REL sub, Oppo 105 (and/or 103), DTV, AppleTV, Sony 4k Media Server.