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 2017-06-16
Hey @Marco we're back to update this idea :)
We've had a chat with the Desktop team and this is still not in current roadmap. However, we've let them know this is a top idea with close to 5,000 votes and asked them to let us know ASAP if something changes on this. They're getting all the feedback and comments here as well. Keep an eye here for any updates, thanks!