I have just switched from an iPhone to HTC M8 and downloaded the spotify app from Google play and I now don't have the equaliser option and my music now sounds poor compared to my iPhone when played through an external speaker. Any ideas ?
There is an EQ setting, it sits under the Music streaming quality settings and is called "Equalizer" (previously known in the app as "Audio Effects")
What devices are you using? Can you try a reboot or re-install of the app? If still missing it may be part of this bug, please go to the thread and post your details.
Android works slightly differently, rather than having EQ's per app/system etc they all tap into a global EQ which controls all volumes, settings globally for all apps that tap into the media output sound channel. The inbuilt EQ (or "Audio Effects" as it's called in the app) in spotify is essentially just a link to the Equalizer built into the ROM, if (as found in some Samsung & HTC devices) the EQ control is missing from the Android Settings then there is nothing for the Spotify to link to, as such it will remove the option.
If this is the case you need to install an app such as ViPER4Android FX (which is what I use - only available if rooted), or Equalizer and the option will appear in the Spotify app. Clicking on it will take you to the EQ app where you can control the settings.