Spotify has an excellent Win32 app for Windows 10, which is even available via the Store. Using the full app gives you access to powerful features, such as seeing what your friends are listening to transfer local music to your smartphone
Spotify also has a basic but passible web player, which lets you access your account and playlists from anywhere at open.spotify.com. This is useful if you want to listen to music at work but are unable to install any software.
Now Spotify appears to be working on adding a new feature. Noticed by reverse engineer Jane Wong, Spotify is testing a new equalizer function for the web app.
The Equalizer appears to offer a variety of presets, and the ability to customize them.
The feature is of course currently unreleased, and it is not known if or when it will become publicly available.