Which surround sound processor $1k used

The Anthem AVM 50 does not support Dolby Atmos or DTS:X, but you can still play those discs through systems for backwards compatibility.  The newer AVM-60 supports them, but the 60 typically goes for $2k used.

Marantz AV770x are very nice processors.  They have excellent power supplies and discrete analog output stages.  Marantz will likely sound more natural and more engaging than other processors, but they are voiced warm with rolled off highs.  That being said, I have been impressed with this processor.  I think the 7703 is likely going to be your best bet.  It's going to be hard to find a good processor with your requirements at the $1k level.

Thanks for your response Auxinput. Seems like Marantz would be great for music but I'm wondering if "warm with rolled off highs" is right for movies. Should I go with a new model from something more mainstream like Yamaha or Denon? Anyone have any other ideas?

I had the Marantz AV8804 in my system for an afternoon.  A friend brought it over to demo/test.  I was actually very impressed with it for audio.  It does have good impact.  However, when I switched back to my Krell S1200U-3D, it was apparent that the Marantz did not have the high frequency extension.  The overall sound and fullness of body with about the same.

I don't think you will get a better HT processor for $1k, especially since you want the latest formats such as 4K HDR and Atmos/DTS:X.  You would have to go to the $2k price point and get a used Anthem AVM 60 to get a faster and higher resolution sound for movies.  I'm not sure that the Anthem

That being said, the Marantz AV7703 is still pretty good for movies.  I think it will be much better than your Emotiva UMC-1.

In my opinion, I would probably slant torwards the Marantz.  The Anthem processors are faster and probably have more exciting high frequency, but I don't think they will sound as natural or as full bodied as the Marantz.