The ability to set individual volume levels per input source is great, as most devices send audio at different levels. However, having to re-adjust the volume for each device is a pain when adapting the volume to different conditions during a ride.
The unit should consider one device to be the 'master' volume level, which one should probably be configurable, but for me would be the A2DP audio source. Adjusting the volume when listening to that source would adjust the volume for all sources.
When listening to a non-master source, adjusting the volume should set the difference between that source's volume and the master volume level. ie: that device's audio is adjusted but no others are. That difference would be preserved until adjusted on that source. This way when the volume is later adjusted on the master source, the other source's volumes are adjusted too, but the difference between sources is preserved.