How to lower minimum iPhone / iOS volume listening to streaming app on bluetooth headset I've looked here 
Reduce minimum

and here
Minimum volume too loud

And did not find a solution to this issue - minimum volume on my bluetooth headset paired with my iPhone is too loud when listening to streaming app like Pandora.

One limited solution that should work with wired or wireless headsets - if you listen with just one earpiece:

In settings select:
1. General/Accessibility
2. Scroll down to Audio Volume Balance slider.
3. Move it towards the ear piece that is NOT in your ear. 
I.e. if you have only the right ear piece in your ear, move the slider towards the left. This will reduce volume in the right ear BELOW the minimum that you can achieve with the regular volume control. Which proves it's possible and why on earth Apple doesn't allow you to lower both channels this way I don't know.
4. May want to enable Mono Audio as well.

Not a solution for hi-fi listening but works well enough for having some casual background music without sacrificing your ear drums.

Has anyone come up with a solution for lowering volume in both ears for streaming audio apps over bluetooth headsets since the other threads were closed?
the " Audio Volume Balance slider " is only a setting to either reconfigure the ear piece so that if one ear piece is more louder than the other. You can change it through that setting or to check if you think one piece is not working this will allow you to verify this, but it is NOT to lower the volume on ear pieces for that you need to press the volume buttons on your iphone or the bluetooth headsets themselves or find the " Audio slider " in the " Audio streaming apps " themselves.   To " lower the minimum volume " is based on personal preference. If changing the volume this way does not work then you may be facing a product quality issue in bluetooth headset because not all bluetooth headsets are created equally