I'm using Spotify on a 2nd gen Motorola Moto LTE, Android version 5.0.2. I'm trying to get an external equalizer to work with Spotify, as the bundled one is close to worthless (as it doesn't allow setting the levels of the bands, just choosing between 4 presets). So far I haven't found a working solution. Most eq's don't affect sound at all. The most promising failure so far has been an app called "Equalizer" from "Smart Android apps llc", which works sometimes, but has a lot of issues (crashing after notification sound has been playing, no sound on phone calls.).
This is what I've tried so far: 1. All the top hits on Play when searching for "eq". Many recommend turning of the "Sound effects" service, this has no effect however, even after much rebooting and so on. 2. Removing and reinstalling Spotify. Resetting phone to factory state and reinstalling Spotify. 3. Disabling 'nuplayer' in developer settings looked promising, but unfortunately it is already disabled.
If you have any other suggestions, I'll be happy to try them.