Does Debian use PulseAudio?
PulseAudio is installed by default in most Debian desktop environments.
How do you use equalizer in Pulseeffects?
To change output or input device, click on the hamburger style menu in the top right corner and click on the Pulseaudio tab. Switch to the appropriate output or input tab according to your needs, and then click on the “Use Default” button to toggle it on or off.