One of the most requested features for Spotify is the desire for a in-app equalizer. There doesn't seem to be any sign that it's coming soon, but there is a workaround, for the mac at least.
It's called Boom.
This little app is a system-wide EQ and volume booster. It has a full range of controls and a menubar icon to help operate it quickly and add presets. It works great to fine tune the music played through Spotify to your speakers, room, and desires. It's also great for quickly changing EQ settings while playing a YouTube video, or a web stream, or any other sound coming from your computer. Its a great little tool. For a limited time, the app is 50% off in the Mac App Store.