Volume on Pass thru line connection to Surround receiver

[Bartype]

Update: As Confused suggested, i have connected something else (a sonos ZP90, configured to control the volume) connected to the line in connection on the Devialet, while configured both normal and pass thru, set to 0 db. When in normal mode, i can control the volume as normal, and combined with the sonos volume this can go very loud.
However, at pass thru, the volume is low (even when set to 0 db). I can set the volume of the sonos to it's max and only then it's playing at what can be considered loud, but not deafening.

I also have set the Devialet pass thru volume to the max possible, +15 db. It can go to higher volumes (but not much) but also starts to sound awful when reaching the max volume of the sonos.

I am not sure what i can do further. The Anthem has always worked perfectly with the Spendor's connected to it directly. And the Devialet is working fine otherwise, and this pass thru option was a great +, but it should work as advertised...

Is there something else i can do?

Thanks again for the tips so far and in advance...