Optimized Center Channel

@soix , it's not as if my system is Bad.  I'm simply trying to go from very good to very, VERY good. It has been stated elsewhere and, as @elliottbnewcombjr pointed out, it may simply be a matter of less than ideal recordings resulting in muddy dialog.  My hope was that someone using an audiophile amplifier for their center channel can weight in as to whether that makes a difference.  It certainly did for my L/R channels.

Yeah, the center speaker works properly, as the dialog coming from it is pretty clean.  Crystal clear so I understand every word?  No, but that could be a matter of chasing unicorns.  I'll play with the phantom center to compare the effect, which seems a matter of removing the center speaker from the setup.

Unless you can come up with a diamond that is large enough, you’re pretty much stuck with WAF syndrome. We all hope someday you win the lottery so you can avail yourself of this curse(Not the wife, just that you will be able to build a separate room for your hobbies). 

I’ve typed this a lot.  I have experience both in theaters and homes and built my own center channel. Here’s the absolute truth:

Center channel speakers will not help you hear dialogue better.  They help you hear the center channel track as if it’s coming from the center of your screen.   If you always sit dead center the center channel speaker does nothing at all for you.   It’s when you sit off center, stand up, sit below that the center speaker becomes important.  

You have a really nice rig, which I believe has custom EQ features.  My suggestion is to sit very close to the speakers.  If the dialogue magically becomes good the issue is your room.  That’s not something fixable by a more expensive center channel.  https://speakermakersjourney.blogspot.com/2024/05/the-center-meaured.html

Also, interesting article here saying it’s really not us, it’s them.  

One reason I know the center doesn’t matter in the center is I have an anthem MRX.  They are all buggy.  They’ve acknowledged they are buggy and lack the previous generation’s feature which is to enable Dolby Surround regardless of the actual incoming source.  Lots of things on Netflix are digitally marked as 2.0 so the MRX will 100% refuse to apply Dolby Surround processing to it but will still tell you it’s on.  End of that story.  I’m glad there are tarrifs on your gear now.  Stupid Canadians... As a result I have the stupid thing double blind testing me all the time.  Effer.  If I’m sitting in the center I 100% can’t tell the difference.  If I’m lying down to the side I 100% can. 

Their previous generation of processors allowed this feature. 

One thing you can have because you have a nice Marantz instead of an Anthem is to use the PEQ features which I think you have to modify the sound track and boost mid bass to mid.  Around 500 to 4 kHz or higher. 

Very nice components within your system. However, when I viewed your system layout, 2 things came to mind. Your center is not optimized, both vertically and horizontally. I suppose left right balance to center can be compensated by Audyssey, but why manipulate the signal more than necessary. I agree with Soix, trying a phantom center, or optimize your center. 
The other is your room acoustics. Interesting test recommendation from Eric. But from looking at your front wall, between the windows and your TV, things will get very lively. I have a similar problem (windows not directly behind, but left & right) with extensive treatment. Good luck. 

@traubr you have an awesome set up for HT.  Congrats on the 16, that’s an end game amp.  I’d be hard pressed to think the issue is your gear.  As others have recommended, running the center in Phantom mode is a great idea, that’s the first thing I would try.  You might really like that change, if the Coda is used in your HT set up to run the L/R speakers the dialogue would now be coming from the best speakers and amplification you have in your set up.  You could also try pulling the center out and placing it more in the room on a stand, then swapping it out for another speaker in that same position.  Leaving it in that position isn’t the solution but it would highlight if the current placement is part of the issue.  You could also disconnect all the channels except the center, listen to the center, swap the center for another speaker.  If you hear noticeable differences would confirm getting another center could be a solution but unless your current speaker is damaged, would be really surprising.  You could also try different amplification, hook the center up to the Coda, see if a different amp changes anything.  Again, doubtful it would but would give you proof of concept before you by new gear.  You’ve tried everything else I could think of as it relates to adjusting dialogue on the Apple TV and also using your Marantz to boost the center channel.  I would also try taking a DVD / Blu-ray disc, compare the disc to streaming the same content, if there is a noticeable difference in how you can hear the dialogue you then will know the compression used for streaming is the issue.  I recently upgraded my Projector to a JVC 4k.  I had streamed 99 % of the movies I was watching, I got a 4 k player and the bigger difference is the sound, not the picture compared to streaming.  Noticeable improvement on channel separation, bass and overall clarity.  When you stream everything is compressed, less data is used due to bandwidth limitations / considerations.  Algorithms are used for both the picture and sound.  Could be that the combo of poor mixing during production of the dialogue and the compression during the streaming process are the combo making dialogue hard to hear.   If that’s the case, only fixes would be using physical media to watch content or buying a high end movie streamer, there are a couple out there but they are expensive and you would still be limited to Movies, content you’d buy through their service, wouldn’t solve streaming Apple TV + shows, Netflix, Amazon Prime Video, Disney Plus or Paramount Plus streaming content.

Good Luck!