So I did some testing, culprit seems to be VLC player somehow:
When switching the VLC "audio output" option from "AudioTrack" to "OpenSL ES", nothing except for a short pause happens, BUT when switching back to "AudioTrack" with VLC enabled in Poweramp "known players", there is no sound at all though the track is still playing. When Poweramp processing is then disabled there is still no sound. Equalizer works only temporarily when 1. selecting "OpenSL ES" output while "known player" processing is disabled, 2. selecting "AudioTrack" as output in VLC and then 3. enabling VLC processing in Poweramp "known players".
BUT THIS ONLY WORKS UNTIL THE END OF THE TRACK! Spotify is still fully equalized but VLC has no sound so that you have to go through the procedure above again, for each track!