Improving the Musicality of a Home Theather

Even cheap HDMI cables can support 4K without too much issue. Maybe not some really old ones.

To justify 4K, you really need a big TV, 75"+.  At 65" maybe if you sit really close you will notice an improvement.

The new OLEDs have blacks every bit as good as your plasma, and much brighter, so better effective contrast in all lighting conditions. As you get older, a big TV can be nice on the eyes too.

Modern TVs have better processing chips so if you watch a lot of sports you may notice an improvement there. The newer TVs also support improved color depth on the HDMI, but not sure you will notice it.

Not sure the best resource. This is stuff I have picked up over the last couple years as I have bought stuff and just generally being into the tech.

https://www.rtings.com/tv

https://www.tomsguide.com/us/best-tvs,review-2224.html

the installer, Audio Breakthroughs in Manhassett, NY did a fancy ARC computer room correction

Great, then you know you are setup correctly.

As for sources it depends on the files you use. If you are using discs (CD,SACD, BRD,UHD, etc) I would recommend a Sony X-800M2. The Panasonic player is nice but lacks the SACD features of the Sony.

If you are using files that you have ripped and stored on a NAS I would recommend the Zidoo Z9X media player, fantastic device,

If you want to stream Netflix, Amazon Video, Disney Plus etc. if you are in the Apple ecosystem get the newer ATV 4K. The Firestick has poor SQ and is crammed with ads. Nvidia Shield is swamped with google ads. The X-Box Series S or X are superb for both streaming and gaming. If you get the Series X you also get a blueray player.

The Anthem piece is superb, you will really enjoy it. As for video I use an Epson PJ in the media room and a Sony 4K TV in the man cave.
After your system is well broken in and you have the sources you like the first upgrade I would do (even before adding two more height channels for atmos) is getting a second sub. It isn’t for the volume of bass, its for the smoothness.

PS- If you add 2 more speakers for overheads move the matching B&W’s from the rear to rear height channels and get matching Revels for the rear surrounds.

You made a very astute decision getting a matched Anthem system (processor and amps), nice job and it seems you got a great audio dealer too.

+1 @thespeakerdude Buy the best LG OLED TV you can afford and you’ll be thrilled.  It also does high dynamic range (HDR) and when it’s available in the program material the picture totally pops — assuming you have the picture settings set right.  Also, if 2-channel is important, upgrade your streamer to something like Innuos, Lumin, or the best you can afford.  The Node is ok, but streamers are critical and moving up to the next level will make a big difference in your stereo performance. No idea about video streaming devices, but glad you’re enjoying your great new system.  

We have Apple TV, Chromecast, Firestick, and ROKU. The "SQ" of all of them is pretty much equivalent and is going to a factor more of what you plug them into. For movie/TV streaming, all the audio is compressed so any jitter on the HDMI is not relevant as it will be buffered and decompressed. The Firesticks are one of the few devices that support Amazon music HD. Not ideal. Bluesound does as well.

Like anything, you have to ensure your devices are up to date. Chromecast added Atmos support last spring. Firesticks added it in the summer. I think the AppleTV added it last spring as well. Disney+ was still having ATMOS support issues till fall of last year. The ROKU ultra I know supports it but it is not hooked up to a full system. The "ads" on the Firestick, other than an ad-bar on the bottom are much like what you get on Netflix home screen. You don't really notice them. The Chromecast and AppleTVs get by far the most use due to ease of interface. No one uses the remotes that comes with these, we all just use our phones.  Realistically, these devices are almost free <<$100, and dead simple to use so little reason not to have one.
 

Android boxes like the Zidoo are popular more with the computer savvy crowd. They tend to require more maintenance, and software can be buggy. We had another brand, but the software updates were infrequent and it eventually collected dust due superior ease of use of other devices. Zidoo has a Z1000 now that is getting better reviews, but more costly.