Are you decoding using an AVR? i think most boxes can do DD5.1, DTS not so common, but i think it's not hard to find one with DD5.1/DTS. If DD True HD/ DTS-MA/ DD Atmos, then those are rare.

I would suggest Mi Box S, probably the best Android TV box (note Android TV is not typical Android box, it has a much better UI and own appstore). I think it can do both DD and DTS.

Bro... you are already on the right track..For your case.. Either the Minix Neo U9-H or HiMedia Q10 Pro will do nicely.. Both of which will easily do even DTS-HD MA nicely. As for the HT connection issue. You can use the HDMI-ARC function for higher supported sound formats and pass-through using Kodi app. I would know.. Works even for the Neo U1.. So for it's successor the Neo U9-H? piece of cake.. Plus, it's been verified already. Since you already have a decent smart TV to access Netflix in its native res and 5.1 DD sound? I say you completely avoid getting the MiBox S and stick with the options above. They've been battle tested and come out tops. Want to "experiment" with something cheaper and report back to us? Get the Mecool KM9 then..

Lastly, your HT isn't an "AVR" per se...Those are usually standalone equipment that aims to receive audio and video signals from a number of sources, and to process them to drive loudspeakers and displays such as a television, monitor or video projector.

Off the top..

Q10 Pro
Older device
Better native HD audio formats support.. Has it's own player and plays nice with AVRS, etc..
Has 3.5in HDD bay for expansion and otherwise purposes.. For media playback. not app installs.
I'm unsure if adoptable storage is supported.
Sometimes buggy Kodi experience.. need to deal with workarounds.
Excellent build quality.
USB 3.0 port and slave available..
Good community and manufacturer support. HiMedia recently released native Bluray menu support.. Not many devices can pull this off even with Kodi.

Minix Neo U9-H
Newer device
Minix are Kodi diamond backers and so Kodi plays so nice with their boxes and fw.
Ships with its own Kodi fork to take advantage of hw & fw.
HD audio support confirmed via Kodi passthrough.
Excellent community and manufacturer support.
Excellent build quality.
Better Wi-Fi cuz 2x2MIMO Dual-Band
USB 2.0
Stable fw
Better apps compatibility.. Think UnifiTV and AOTG.. etc.. Install or update from official channels.. set and forget..
Adoptable storage... Means you can add a drive and merge with the internal drive..Samsung EVO 256GB microSD card is tested and confirmed to be working on NEO U9-H

Can't recall anymore at the moment.. If you ask me which to pick between the 2? I'm a Minix guy all the way.. But up to you.