well... if u do not play bdmv, it's irrelevant, no need.

if u do, that is   

stable, i already using it.

firmware 2.0.6 has only one major change from 2.0.5 - 4k full bluray menu.

works good for 1080p and 4k BDMV, BD ISO.

no idea about 3D ones.

Get the Himedia Q10 pro . i was in the same boat as you and trust me, the current state of media players in the market is pathetic....

this is something i hv been telling my frens and those interested who have a good collection of old stuff.
with a 4k tv, all of them look different - colors, clarity, smoothness.
with old tv+q10pro or new 4k tv+old box, u surely wun get that.
i just watched a 2013 12gb 1080p movie... hmm... so many new colors, deeper colors, new details.
what box is this? your old himedia or something else?

kodi 17.5 (not earlier 17.x) allows ac3/eac3 audio pass thru, so 5.1 is not surprising.
those streams from megabox, showbox are typically poor ones; good if there are some aac 5.1 there now.
and if megabox app player can't do it, it means it is indeed poor!
try terrarium 1.8.1 - their new "evo" player is quite good, can do aac 5.1 if the streams are such.
or u can use kodi player or himedia video player for 5.1, same result.

now, if u use kodi player for such a purpose, u r better off using kodi quasar addon direct.
i do not know which boxes can or cannot do with 17.5 at this time, but q10pro can do all dd/dd+5.1 streams.
can even do uhd 4k streams but audio will be just dd/dd+5.1/7.1.
if original audio is dts/dtshdma/ddtruehd/ddatmos, it will be transcoded to just dd5.1/7.1.

the day will come the better android boxes will be able to stream online uhd4k+dtshdma/ddtruehd.
will need pretty high speed like >100mbps, of course.

q10pro... firmware 2.0.8 is out.

wonderful news for some...

let me test further, reconfirm all is good first.