since there is still no solution on the part of Spotify I think it is time to Open a New thread to this issue in case the old one is set to "fixed".
Many others faced the exact same Problem since updating from lollipop to Marshmallow. Even people who made a complete New Installation of Marshmallow on their phones have it.
The audio quality is broke massively while using EQ with surround Sound turned on. The frequences get distorted, Songs even seem to pitch up and down while playing. The only thing to prevent this issue is turning off the whole EQ or disabling surround Sound. But everyone knows, that the quality is a mess without EQ when using headphones.
I run a Nexus 5 with Marshmallow. I tried both scenarios: updating OTA and a completly fresh Installation of Android. The problem isn't gone.
Please fix this issue soon! There have been three updates of the App since Android 6.0 was released and still we didn't get this Problem solved.