[Q] Bluetooth Headset - more Volume Steps topic

Hey,

I bought a Sony SBH20 to listen to music without having the annoying cable on my phone.
The Problem is, there are only 7 Volume Steps for the Bluetooth Device (Hardware Buttons of the Headset also control the Android Volume).
So I want to find a way to controll the Volume finer.

I tried:
- Increase number of steps with the XPosed module VolumeSteps+. Problem ist that also I have more steps now, only at 7 points the Volume realy changes. So I have no improvement here.
- The App "Fine Volume Controll V2.0" which can controll the Volume in 1% steps. Problem here is that I can only change the Volume in the app, but not with the Hardware Buttons of Headset or Smartphone (when the phone is locked).

So, any ideas for that?

My Phone is a rooted Sony Xperia Z2 with Android 4.4.4 and Xposed Framework installed.
I don't have this Problem with my old Galaxy Nexus with CyanogenMod (also Android 4.4.4) which has more steps for the Bluetooth Device. So I think the Problem belongs to Sony customizations of Android.

So is there any way to get finer Volume controll?