I have a 37pfl7605 TV on HDM 1 I have connect the HTS9520, on HDMI2 I have connect a SetTopBox and on HDMI 3 a Dune HD Smart D1 media player
As the TV does not support DTS I've connected the to the DUNE via an optical link to the HTS.
The Dune has CEC functionality.

When the Dune is active (not standby) everything works fine. The Dune appears in the home menu and when I switch to HDMI 2 (my STB ) the sound plays via HDMI ARC When I switch to HDMI3 (Dune) the sound plays via TosLink. Perfect!

When my Dune is in standby and I switch to my STB the sound is played via the internal TV speakers and I can not switch to HTS even not when I manually switch to external amplifier (TV option menu)

So the system works only when all devices are on. Is this common behavior?