There should be an Equalizer for the Desktop version of Spotify.
Since Spotify's mobile apps now have an EQ setting, this should be implemented on the desktop version, too!
The EQ should come with various presets but also the ability to save your own custom settings.
Updated on 2022-08-10
Thanks for coming to the Spotify Idea Exchange and voting to make this happen.
We're happy to announce that the Equalizer option is now available on desktop - and is gradually rolling out to all users over the next week. You will be able to play around and customize the sound of your music for a better listening experience by toggling your Equalizer on your computers.
Once again, thanks for helping us make Spotify better, by participating here on the Community.