Hi,
if soundbar has several HDMI in I would connect soundbar and TV with the ARC connection and connect STB and/or other devices directly to the soundbar. The reason for this is that not all TVs send surround (5.1) from external devices via ARC to an external AVR/soundbar.

As for HD sound and ARC watch for eARC which is/will be the next generation. My Denon will get a firmware update for this.

ciao