Impossible to control mp3 cbr quality in batch mode

I have read all the answers provided related to "quality" in SoundConverter.

My use case is as follows:

- I am converting from m4a to mp3;
- I have set the CBR and quality "insane" in UI of SoundConverter version 3.0.0 (which is 320 kbps);
- everything works as expected through the UI;
- when I try to convert from console using batch processing -b setting for soundconverter: the mp3 is generated with VBR instead of CBR;
- I am using XFCE (Ubuntu 18.04 and Debian 9), and have set in gconf editor the keys:
    -- gconftool-2 --set /apps/SoundConverter/mp3-mode --type=string cbr
    -- gconftool-2 --set /apps/SoundConverter/mp3-cbr-quality --type=int 320
- still, with those settings I get an mp3 with VBR quality;
- I am generating from console using the following command:
soundconverter -b -m "audio/mpeg" -s ".mp3" input.m4a

Please help.
Many thanks!