Xiaomi Mibox 3 / S - abt 6K on amazon.in, same as Shield interface (Android TV interface).. It can bitstream Atmos to AVR..
I use Nvidia sheild and my brother uses Xiaomi Mi box 3, in both scenarios i have used it to FHD capability only.. Never played 4K content as we both are still with FHD capable projectors..
I had used a generic Android box.. chinese ones, it was able to do the duty of FHD playback & Atmos bit streams.. This is enough if FHD and Atmos bitsream capability is enough.. No experience with 4K content..
Yes IMO.. All you need is capable media player.. All my experience has been with Kodi so far and it gets the job done..
Anyone would need application updates.. I think OS update will not apply here..
The generic Andorid box that i had was Android 6.0.. Never got an update..
Nvidia / Xiaomi Mi box - Both were 7.0 out of the box and now stands at 8.0.. In all these scenarios, it was Kodi doing the audio bit streaming and it did it, regardless of OS update..