After a 2-day struggle to figure out how to stream Dolby Atmos successfully from Netflix and get it fed to the A16, I have finally emerged victorious. I did have to reconfigure and recable things significantly, but in the end it is all now working perfectly. As I'm sure it would probably be helpful to others I will share what I had to do given the equipment I had, and you can make your own adaptations as appropriate for your own situation.
(1) I swapped out my 6-year old Yamaha RX-V867 for a "recent vintage" (i.e. 2-year old) RX-A860. The A860 supports 4K video as well as Dolby Vision and Dolby Atmos, none of which was supported by the V867. I don't have a fancy "real" multi-speaker sound system (which would utilize the 7.1 main speaker outputs of the AVR), but rather only have a good 2-speaker stereo setup for when I want to casually listen through speakers instead of headphones. All Yamaha AVRs support a separate stereo Zone 2 activated by the "party mode" button on the remote and managed by a separate Zone 2 volume control, and that's how I deliver just perfectly acceptable 2-channel stereo sound to these two speakers which are connected to the Zone 2 speaker outputs of the AVR.
Note that "party mode" delivers a pure stereo down-mix of the main multi-channel inputs to the AVR, but with only a primitive bass/treble tone control. However ideally I want a more sophisticated "tone control" applied to the 2-channel stereo sound from the speakers which is not provided by the AVR. So for this I use a 14-band vintage 1987 DBX 14/10 graphic equalizer. The EQ has two analog stereo inputs (ostensibly for SOURCE and TAPE), which I use instead to accept two separate analog inputs coming from L/R preamp outputs of my Oppo 103 and Oppo 203. The Oppo players also support 2-channel analog stereo down-mix of multi-channel source, delivered to the L/R analog preamp outputs, and I then feed to L/R output of the 103 to SOURCE input of the EQ, and the L/R ouptut of the 203 to TAPE input of the EQ. The 14/10 EQ supports 10 memory EQ presets (each of 14-bands), but as with most setups I only use one preset (i.e. "set it and forget it") which just "sounds the best" no matter what.
The single L/R analog stereo output of the EQ feeds the AUDIO1 analog L/R input of the AVR which is assigned (through INPUT setup on the AVR) as the audio input for both HDMI1 source (fed from Oppo 103 HDMI1 video-only output) and HDMI2 source (fed from Oppo 203 HDMI1 video-only output). So depending on which Oppo player is providing source content that I want to listen to through speakers, I power on the EQ (already selected to my "favorite" EQ preset) and push either the SOURCE or TAPE button on the EQ, and push the "Party Mode" button on the AVR remote.
(2) Turns out the Roku Ultra streamer is significantly inferior to the Apple TV 4K. The Roku device does NOT support Dolby Atmos, and also does NOT support Dolby Vision!! Hence its Netflix app delivers best-quality video at 4K HDR, and delivers best-quality audio using Dolby Digital+ (e.g. 5.1). So this is how "The Irishman" shows on the program info screen.
In contrast, the Apple TV 4K supports BOTH Dolby Atmos audio as well as Dolby Vision video. So both of these logos appear on the program info screen using the Netflix app of the ATV 4K.
Complicating things significantly, it turns out the Oppo 203 does NOT support Dolby Atmos audio pass-through through its one external HDMI input. A firmware upgrade a while back added a special "HDMI IN BYPASS" to allow Dolby Vision source through its external HDMI input, which is simply passed right on out the HDMI1 video-only output with zero processing by the 203 (also meaning that the onscreen displays of the 203 are completely unavailable when "HDMI IN BYPASS" is selected). However this firmware upgrade didn't affect the non-support for Dolby Atmos audio through the external HDMI input.
This made it very challenging to support my two Oppo players along with (a) the Roku Ultra streamer I already had, (b) the Apple TV 4K that I just got this week, and (c) the two Linksys DMA2100 Windows Media Center Extenders I use to deliver 720p/1080i HDTV from my two Win7 HTPC's. And of course the real goal was to be able to utilize the A16 for Dolby Atmos content, either (a) played in TrueHD lossless from new 4K UHD BluRay discs via the Oppo 203, or (b) played in Dolby Digital+ lossy from the Apple TV 4K.
So, how to cable everything I own and have it work properly? Turns out the secret is to make use of the multiple HDMI inputs of the A16, as well as the HDMI output of the A16 (something I had not made use of until now).
(3) Here is the configuration and cabling for the A860 AVR:
(a) HDMI1 input ("BDP103 WMC FHD") fed from HDMI1 video-only output of BDP103, with analog audio input AUDIO1 coming from analog L/R preamp output of the 103 (stereo down-mix in speaker configuration) routed through the DBX 14/10 EQ "SOURCE" input. This supports 1080p video from the 103 disc player (and its two external HDMI input sources which are the two DMA2100 WMC extenders), with audio tone control for the optionally used stereo speakers from the EQ.
(b) HDMI2 input ("UDP203 ROKU 4K") fed from HDMI1 video-only output of UDP203, with analog audio input AUDIO1 coming from analog L/R preamp output of the 203 (stereo down-mix in speaker configuration) routed through the DBX 14/10 EQ "TAPE" input. This supports 1080p/4K video from the 203 player (and its one external HDMI input source which is the Roku Ultra), with audio tone control for the optionally used stereo speakers from the EQ.
(c) HDMI3 input ("A16 A/V ATV 4K") fed from HDMI OUTPUT of A16, with digital audio input also from the A16 via HDMI3. This supports 1080p/4K video from the ATV 4K (including Dolby Vision), as well as digital audio (including Dolby Atmos to headphones via the A16), as well as optionally used stereo speakers but without DBX 14/10 EQ tone control.
VERY IMPORTANT NOTE: the Apple TV 4K firmware upgrade added two new Settings video options which are delivered in an OFF state by default: (1) "match dynamic range", and (2) "match frame rate". These two options should both be turned ON. This allows automatic proper video interfacing to either a 1080p TV or a 4K TV, along with the correct enabling of HDR or Dolby Vision, etc., depending on the capabilities of the connected TV. It also automatically lets SDR content to be viewed in its ideal native SDR form, rather than being forced up-converted to HDR.
A direct result of enabling these two video options is that when looking at the Netflix info page for "The Irishman", when the ATV 4K is used along with a 4K TV (like my 55" LG OLED C7) both the Dolby Vision and Dolby Atmos logos are shown. But when the ATV 4K is used along with a 1080p TV (like my Panny 65VT50) the audio is still shown as Dolby Atmos but the video is now shown as HD. This is perfect, and now automatically done by the ATV 4K itself thanks to setting those two video options to ON.
(d) HDMI1 output feeds my 1080p Panasonic 65VT50. Note that I have purchased a new 4K 65" LG OLED C7 to replace the Panny, which will be here and installed on Friday.
(4) Here is the configuration and cabling for the A16:
(a) HDMI1 input fed from HDMI2 audio-only output of BDP103.
(b) HDMI2 input fed from HDMI2 audio-only output of BDP203.
(c) HDMI3 input fed from HDMI audio/video output of Apple TV 4K.
(d) HDMI audio/video output feeds HDMI3 input (for audio/video) of the A860.
(5) For viewing and listening convenience, I have established NINE presets on the A16:
(a) 1-3 for content from BDP103 on HDMI1 (including both DMA2100): 1 (5.1 PCM), 2 (7.1 PCM), 3 (9.1.6 Dolby Atmos)
(b) 4-6 for content from BDP-203 on HDMI2 (including Roku Ultra): 4 (5.1 PCM), 5 (7.1 PCM), 6 (9.1.6 Dolby Atmos)
(c) 7-9 for content from Apple TV 4K on HDMI3: 7 (5.1 PCM), 8 (7.1 PCM), 9 (9.1.6 Dolby Atmos)
And it works! Dolby Atmos on A16 via both UDP203 (for new UHD 4K discs), as well as streaming via Apple TV 4K.
Thankfully, the A16 HDMI 4-input 1-output pass-through capability solves the problem caused by the inability of the Oppo UDP203 to accept Dolby Atmos input (from the ATV 4K) via its external HDMI input. I haven't had a chance to really use the new setup yet, so I don't know if there will be a lip-sync issue needing some adjustment to the settings in the new A16 lip-sync functionality.
Again, the Roku Ultra does not support either Dolby Vision or Dolby Atmos. This makes it largely no longer of much value to me as it largely duplicates the "important" main streaming sources like Netflix, Amazon Prime, Hulu, etc. which are also available on ATV 4K. Only some additional cable and on-demand premium TV channels (mostly non-4K) that it supports uniquely are not available through Apple TV 4K but only through Roku. I suspect my use of Roku will mostly disappear.
I hadn't really expected to have to purchase an Apple TV 4K (32GB is good enough for me), but that turned out to be the only way to stream Dolby Atmos. The Roku simply doesn't have the licensing. Took me about 30 minutes of flailing around with the remote to finally accidentally stumble into the fact that the remote actually has "invisible arrow keys" and a "push-to-ENTER" on its touch-sensitive upper portion that is otherwise purely flat metal with no markings or indications that this is where the crucial cursor control and OK "button" are located. Also, pushing the MENU button is like "back". I was quite surprised to discover ZERO INSTRUCTIONS OF ANY KIND in the carton to provide at least a few basic guidelines for using the remote. And being a 100% illiterate user of any Apple product I don't know if this is really consistent with other Apple remotes (so that if you're already part of the Apple ecosystem you would immediately intuit how this remote works). But for me, I had absolutely no idea how to even select my language of English from what I was seeing on my TV screen! Anyway, I'm now duly educated.