TV audio into DAC via eARC or better

Bluesound Node N130 or X has eARC and USB out. If you are buying such an expensive DAC, the cost is trivial. Plus, they are streamers, with a well proven app.
This was mentioned earlier, but needs to be repeated. Get a Node!  Another point…they have an auto switch that engages when the eARC detects a signal, and a 12V trigger output, so if your other gear is equipped with trigger inputs, you can automate the whole turn-on sequence.

“Probably going into a new DAC in the $10k-$15k range.  Maybe MSB Discrete ”@chauncey 

Your original post suggests, you would like to improve TV sound which includes movies plus other content. How is connecting a MSB DAC or Node with eARC would let you enjoy Dolby Atmos soundtracks? What I am missing here? Even optical cable/connection won’t transmit Dolby Atmos???

”The eARC (Enhanced Audio Return Channel) enables you to transmit the original full-resolution audio signal through an HDMI cable and reproduce the best sound without compromise.” 

My HDMI extends from my Apple TV 4K to a Nad m17 processor, and then I have RCA interconnects from the processor to a Conrad Johnson gat2 preamplifier which is connected to amplifiers.

Of course everything flowing through Xfinity is subpar. But all the streaming stuff going through Apple TV for nonstereo listening is hard to improve on given inherent streaming Quality deficiency.

the power cords, HDMI cables, Ethernet cords and network switch are all upgraded.

What more can be done here? Sound is quite good on my system but unfortunately it's entirely dependent on the quality of the source. And streaming Quality is inconsistent and always compressed. 

If the Apple TV 4K and Nad M17 both have eARC, streaming should not be compressed.  You need to assure both have eARC.  After the processor it's analog.   Of course audio quality is dependent on the source, be it vinyl, disc, or streamed.