Suggestions for replacement for my old Nuforce AVP-18 surround sound processor

First thing.  Dolby Atmos and DTS:X surround formats are fully backward compatible.  That means that you can play an Atmos movie on a system that does not support Atmos and the movie/sound will play just fine.

Second, the newer 4K processors are only good if you have a 4K source.  Some of the streaming sources will have 4K.  So it's up to you as to whether or not you really want to support 4K.  Some of the newer processors may have some additional video processing chips that could help.  I'm not sure how much. 

If you want both 4K and Atmos support, probably the best processor for the money is Marantz AV7703 (or higher).  Marantz is an excellent processor with discrete analog stages and great power supply, but it is voiced just slightly on the warm side with rolled off high frequencies.  You should be able to get one for under $2k no problem.  They are on ebay all the time (AV7703 / AV7704 / AV7705).  They support the latest HDMI 2.0 and HDCP 2.2 standards.

The AVM 60 is going to have much more high frequency response and could sound a lot more exciting, but it could come across a little bright and I don't think it will have as much fullness and body as the Marantz.

If you want to remove your 4K or atmos requirements, there are a lot of options.  Such as Classe SSP800 or an Anthem D2v if you can find one.  Heck, even the $1k Outlaw Audio 976 processor is not bad (I've tested it).

The Nuforce AVP-18 is really a bare bones processor (switching power supply and SMD op amps for output).  There's hardly anything inside and you have a LOT of options for a significantly better processor.

Thanks Auxinput.

Yep the avp18 is a basic boogie box for sure but it has been/ is reliable and has done its job.
Will be looking up the Marantz models although they seem huge and bulky and I do like the slimline look of the avp18 I must admit.
We do have 4k tv and 4k streaming service and not into the Atmos right now but always nice to have options.

For reference, there's an internal pic of your AVP here.  It actually looks very similar to the low-end processors that Emotiva has offered over the last several years:

https://www.audiosciencereview.com/forum/index.php?threads/the-rise-of-audio-negociants.14384/

Generally speaking, slimline and audio quality don't really go together, especially on processors.  There's a reason for that.  It takes space for a good transformer linear power supply as well as all the DAC and audio outboard board circuits.  I cannot find a good pic of the 7703/7704/7705, but the Marantz is packed full of layers of circuit boards.  The Anthem AVM50 is not quite as big, but it's still larger and full of boards.

Probably the best "slimline" type of processor I know is the Krell Foundation 4K, which is excellent, but now you're looking at $4.5k used.  It's only 3.5" high!

Auxinput
Funny you should mention Emotiva as my initial thoughts were that the AVP18 was just a recased Emotiva as the menus are EXACTLY the same as found in Emotiva processors( I owned two previously!)

Also the Outlaw 976... one for sale right here right now at $699......

Yes I get the slimline is mostly because there is nothing much in the AVP18....lol
But the Marantz just is too big and I have not checked but it might not even fit in my rack where the AVP18 lives right now.

$4500 is way out of budget, especially for something that is still working quite fine atm. 

The Outlaw 976 actually shoots higher than the price, in my opinion.  It uses linear power supply.  Has very good op amp circuits and LOTS of localized power supply capacitors surrounding the opamps.  The XLR output stage is fully balanced/differential.  It has a pretty good fullness of sound and the audio actually sounds very natural.  It doesn't have the best high frequency response and the bass is not the strongest, but it's probably close to the sound quality of a Marantz AV7703 in my opinion.  The Marantz has this thing beat, though, because Marantz has much better power supply and audio circuits.

I actually completely rebuilt a 976 for a friend, upgrading critical op amps for left/center/right/sub as well as completely re-capping the entire audio boards.   Replaced all op amps on the XLR output board as well.  Probably about 140 total parts.

It is engineered to be a low price product and it shows with the user interface.  It's cumbersome because it blanks out the screen completely for a couple seconds when you go into the menu.  I think it treats the "menu" as a different source.  The menu does not overlay the current video like most other processors.  But it is functional and it works.  Has a fully parametric EQ - you can dial in exact frequencies.  No automatic room correction (if that matters to you).