I'm having the same problem.
I have a pioneer 7100nex and about 2 weeks ago I had issues with my Note 4 connecting to the pioneer unit where the screen would go black. It wasn't the Head unit it was the phone. The volume via usb was normal and had not issues.
Ever since I reset my phone, my volume is extremely low. I've usually had it about 5 or 6 volume and now have to bump it to about 20 to get decent sound.
I checked Google play equalizer settings a maximum and even activated the adapt sound and had the default volume to 15.
But when plugging the phone back into the usb I still get the same low volume.
I even changed USB cables.
Is there a way to fix this or can anyone confirm what setting on the phone I need to use?
My SD card and bluetooth work fine with the volume levels.