How to disable/bypass Amarok's (or Gstreamer) volume control for audiophile sound

[calabuig]

Hello,
I'd like to use raspberry pi 3 + ubuntu for an "audiophile" sound. I have a m2tech Hiface Evo card that is connected to an external DAC.
I tried to use mpd + alsa with good results, but I would try to compare it with the Amarok player using Gstreamer phonon.
Now, the audiophile sound provides, among other things, the disabling of the volume. Hiface hasn't a hardware or software volume control. In the mpd's config, however, I disabled the mixer with the value "none" and so I am sure I haven't the volume control (in various mpd's front-ends the volume bar is either disabled or has no effect on the sound).
I would like to achieve the same result with Amarok and Gstreamer, but I can't find the right command on the web documentation.
The Amarok's volume control has effects on reproduction. I precise that I have disabled Pulse.
How can I disable/bypass the volume on Gstreamer (or Amarok) settings?
Thanks in advance!