Do this go in to settings audio advanced tweeks uncheck direct volume control for both direct and bt this will increase your volume to where it should be took me a bit to figure out but it fixed my poweramp low volume issue
You are a genius. I have seen those two settings for direct volume control and Bluetooth direct volume control, but I have never realized that could be the fix for the low volume problem. I did exactly what you said and it fixed the problem immediately.
I am using a "ZTE Grand X Max 2" telephone with OS version marshmallow 6.0.1 . My Poweramp version is 2.0.10-build-588-play with no plugins installed.
Just so everyone knows, having these two options enabled can also cause the "Replay Gain" (RG) features not to work.
Also, if you have these two features enabled you will notice that the bass level decreases as you turn the volume up and the sound will become distorted when you turn the volume up to maximum. So it is best just to disable these two features all together.
Thanks again to the person that posted the answer which is shown above, you have solved multiple problems with this fix.
On another note, there is a separate issue that I think people here should know about. Whether you have DVC enabled or not the following issue will occur in version 2.0.10 or later. When using The Equalizer in Poweramp, if you try to increase the level of bands 2K, 4K, 8K, or 16k it will cause the treble knob to stop working. It seems that this is some sort of a glitch. The current Poweramp version that is available as of this comment is version V3 build 703 Alpha. In this latest version the issue is still not fixed. A workaround is to turn the "tone" button off and strictly use the equalizer to change the bass and treble.