On my iPhone 7 (latest OS update), the EQ has a really bad defect that makes it impossible to listen "Flat".
Very easy to reproduce:
1 - Set the equaliser to some mad settings (e.g. no treble, loads of bass)
2 - Switch the EQ completely OFF - and listen carefully to the sound
3 - Set the equaliser to opposite mad settings (e.g. no bass, loads of treble)
4 - Switch the EQ completely OFF - and listen carefully to the sound
Since the equaliser if OFF in steps 2 and 4, the sound should be identical - it is not. Not even close! It seems like the EQ OFF switch is leaving some bands active.
In steps 2 and 4, selecting "Flat" (which also switches off the EQ) doesn't help either - some bands are very audibly still boosting or cutting despite the graphic showing a flat line.
Editing the "flat line" response as part of this experiment is revealing; as you *slightly* move the bands on the flat line, you can hear them suddenly "snap" back to neutral - so clearly they've got out-of-sync with what the UI is showing.
Would be good to prioritize fixing this; otherwise I can't trust what I am listening to on Spotify - which is a big deal for the work that I do.