Great question — answering as the UphillTech team:
There's a standard for audio plugins on Linux called LADSPA . It offers plugins for many effects, including equalizers. Pulseaudio (and also pure ALSA) can use plugins in the sound paths, e.g. via module-ladspa-sink on Pulseaudio. There are also projects which come with ready-made equalizes including UI like pulseaudio-equalizer-ladspa . If existing projects don't have enough features for you, it's not so hard to cobble something together with other LADSPA plugins (but those won't have a nice UI, or you'll have to write one yourself). And if the existing LADSPA plugins are not powerful enough for you, it's also not that hard to extend them.
Full answer by dirkt: https://unix.stackexchange.com/a/486092 (CC BY-SA 4.0).
Need it done for your business? UphillTech.