To support DolbyVision to the TV (LG C9 OLED), all my sources go to the AVR and then I take the output of the AVR and run it through a
4K splitter, with one output feeding the TV, and the other feeding the A16 with a 1080p down-res of the stream so it can perform audio processing via HDMI1.
I don't understand the purpose of the down-res to 1080p for that split output feeding the A16. What is it that this enables in the A16, that would otherwise not be possible? If I simply place the A16 between the AVR HDMI output and the TV (such that the video would be 4K passing through the A16), why is that a problem? Is this because the single HDMI output of the AVR can't support two full 4K@60hz outputs via the splitter? If that's the answer, rather than something caused by the A16 itself, then now I understand..
Note that not having the A16 connected to the TV prevents ARC/eARC audio going back from the TV to the A16 when using apps on the TV, if you wanted to do that.
Also, as I thought more about my earlier comments I reminded myself of some real complaints I had about the initial implementation of the new "HDMI pass-through in standby mode" feature now available with 2.00+ firmware. At the very least, the ST/HT LED's for the head tracker remain on even when the A16 itself is in that semi-off reduced power state that keeps the HDMI sub-assembly powered but cuts power to everything else. Well they forgot to cut power to the head tracker circuits (assuming it is actually possible to selectively cut power to the head tracker items), which are obviously totally irrelevant to HDMI pass-through. Also, the display screen has the message about pass-through enabled for the last-used HDMI input permanently displayed while in this special standby state, and there's no way to make the screen go fully dark instead ir that's what you preferred.
And finally, they should have implemented a 2-step HDMI standby process, so that it doesn't have to remain in the one state permanently. Rather, you could just have in in HDMI standby state for as long as you were watching something and listening through speakers. But when you were done, and wanted to turn off all the lights and go to bed, you should have been able to push POWER a second time and now the A16 powers down 100%, fully disabling the HDMI pass-through facility. And then the third press of POWER brings the unit back to normal life. Unfortunately this is NOT what happens, and instead pressing POWER that second time actually powers the A16 fully back up! You've just turned it back on, although you really wanted to leave the room for the night. The only way to actually put the A16 to sleep is to get into Settings and disable HDMI pass-through, and then power it down!! Ridiculous, of course.
So, in the absence of an upcoming firmware upgrade to resolve these problems, I think the best compromise for now is NOT to set HDMI standby enabled normally. Keep things as they are without that feature, most of the time. And only if you have that special occasion where you'd actually rather listen through speakers would you then activate that setting, and de-activate it when you are finished with that one viewing session. It's unfortunate we have to compromise but for now I think leaving it OFF is the lesser of two inconveniences.
On the other hand... maybe that splitter idea is REALLY the best idea.
One way or another, simply passing the HDMI output from the AVR through the A16 is clumsy. Either you have to power the A16 on to pass through video even if you were listening through speakers, or you use HDMI standby and keep it "turned off" but with all of those other negatives I enumerated.
In contrast, the splitter trick allows you to keep the A16 powered fully off when you're not using headphones. Period. If/when you want to use headphones, you power it on as we've always done. It's much like I have right now from my Oppo 203 which runs essentially in "split-A/V mode" (which is what the 103 calls it). With the 203 HDMI1-OUT is for video only and HDMI2-OUT is for audio only, if the device on HDMI2 is powered on. If there's no active device on HDMI2-OUT then HDMI1-OUT sends both audio and video. Since my Yamaha AVR only 2.0 audio out its HDMI2-OUT, the only way to get full multi-channel HDMI audio to the A16 would be to use HDMI1-OUT running through the splitter, as you suggest. But the A16 is really totally independent of the video going from the same HDMI1-OUT of the AVR to the TV.
I still have that Monoprice matrix splitter I bought last year and never used. I think I maybe could make use of it here. But I will look into the one you use.