Well this is the complicated part.
I have Sony HT-S20R which is 5.1 supported. Even if I tune to any SD or non-DD channels, Sony will sort of convert the audio to 5.1. I can confirm this as whenever I go near the rear surround speakers of my soundbar, I can hear the surround sounds or dialogues or audio.
The problem starts when I tune to HD channel with DD/DD+ where the volume drops by but after increasing the volume, it becomes normal.
Regarding audio quality, I feel it is same however sometimes I get better sound from 9XM rather than MTVBeatsHD.
However, I am still unable to find any option to be sure if 5.1 input is coming from STB or soundbar is doing its upscaling.