Owned one RT5 since November'15, waiting for other to review if the same 'issue' i have and i see the same here - Volume is slow even at max for HDMI inputs (specific AstroHD) but fine with Bluetooth input.

Connection - Astro HD -(HDMI)-> RT5 -(HDMI Output)-> TV (Samsung).
Not using ARC on this TV.

Literally to compare on HDMI inputs, my Samsung UA-50ES5500 TV is louder than this RT5 at max volume. (i think 20W vs 550W?).

Changed HDMI cable to no difference.
Explored bringing up the volume from Astro Decoder itself but no change - it shouldn't anyway as the audio is transmitted digitally.
Explored Optical but understand will loose DD, which is available on HD channels i watch the most.

I plan to get another HDMI input (i.e. blueray/dvd) to isolate if this is specific from the AstroHD Decoder box but any other ideas without investing on this blueray/dvd that i seldom will use?

Its not a major issue as it is clear enough at 35-40 volume levels, but i don't see a major difference of dB either at max 50 volume level.

Thanks! So there is still hope in Optical. need to research this further if Astro Byond decoders support DD. Anyone?

Yes - have switch to Digital Audio in Astro Byond as here