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

Emotiva XMC-1 for $950?

https://www.ebay.com/itm/Emotiva-XMC-1-Gen-II-7-2ch-AV-Preamp-Processor-all-accessories-original-box...

You can get an upgraded HDMI 2.0 board for this for $399, so that you can support 4K if you want:

https://emotiva.com/collections/processors/products/hdmi-2-0b-video-upgrade-board-for-the-xmc-1

Just a thought.  XMC-1 has 5-3/4" height. 

Marantz 7702 has a 7-3/8" height. 

Outlaw 976 has 4-5/8" height.

Krell Foundation has 3-1/2" height.

Nuforce AVP-18 has 3-1/5" height.  (basically the shortest height)

Sorry you have to look to the more expensive Krell Foundation.  Just keep in mind there are two potential models.  There is the original version, which only supports HDMI 1.4, and the "4K UHD" version.  A lot of times the original version sells for much less, but can be upgraded by Krell.

Be aware that Integra 80.3 is 8 years old and does not support HDMI 2.0 and 4K HDR.

You indicated that you have a 4K HD television.  If you want to be able to streaming 4K content and sources, you will need that HDMI 2.0 support in a processor.

The alternative is to connect your source directly to the television, but then you have challenges on how to get the audio signal into the HT processor.

That being said, you don't necessarily have to have a processor that supports HDMI 2.0 and 4K.  You can play 1080p video through the processor and TV.  The TV will still accept the signal and scale the image to the native 4k resolution.

Using a digital input from bluray player can be done, but it really depends.  I have seen some processors having a problem when you have Dolby Digital Plus.  Sometimes, the only way to do Dolby Digital Plus is over HDMI on a newer type processor.

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!

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.

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).

yeah, unfortunately, me and many others think that Krell failed big time on the asthetics of their current line.  The previous Evolution line looked a LOT better, much like a Mercedes.  The current line looks cheap, like a Toyota, but the build quality is still excellent.

I keep forgetting about that processor, mostly because it only has balanced XLR outputs for left/right, but since you are only going to use RCA interconnects to your amp, it shouldn't matter to you.  The Sigma SSP uses the same audio stages as the previous SSP-800 (which is a fully balanced design).  The Sigma is only 3-3/4" high, which is excellent for you.  However, the Sigma SSP also had two versions just like Krell Foundation.  The original only supports HDMI 1.4 and 1080p.  The newer "mkii" version will support HDMI 2.0 and 4K UHD and some limited Atmos decoding.  Here's a "mkii" version for a little more at $2799:

https://www.audiogon.com/listings/lisa4f8g-classe-sigma-ssp-mkii-7-1-channel-home-theater-processor-...

That being said, the Sigma SSP uses switching power supply for everything.  Since your AVP-18 also uses switching power supply, it's probably not a big deal for you.  The Sigma SSP will still be a significant upgrade from your AVP-18 in all areas.

I think the Krell Foundation will still have better sound quality.  One of the things I really like about the Krell design approach is that they use linear power supplies for everything, including both digital control circuits and DAC VREF power supply.  Most processors will use a linear power supply for analog sections, but still use switching power supply for digital sections.  I think the Foundation is going to sound smoother, fully and more natural than the Sigma SSP, but it's going to be $1700 more, lol.  The Sigma SSP will still be a huge improvement over your AVP-18, though.  I guess it depends how much you really want to spend, lol.