Direct Volume Control - or DVC - is an option that provides a cleaner and more uninterrupted audio path from Poweramp to your output device, removing some of the interim stages that the hardware manufacturer may have incorporated into their design. Because the hardware volume is controlled more directly by Poweramp, there is generally an improvement in dynamic range and more headroom for equalisation to take place so you can safely boost the bass or treble further - both of which may also result in an increase in possible volume levels.
DVC (if available for your device) can be activated using PA Settings -> Audio -> DVC -> Enable Direct Volume Control. Normally that would be the original default setting, but some devices or output combinations do not support DVC at all, see below for examples.
The control beneath that, No DVC Headroom Gain is a gain reduction setting that is only called into use whenever DVC is not active, to provide better EQ headroom without distortion by slightly reducing the final volume level. If you are currently using DVC though, that control doesn't do anything.
It is also possible to override these two global settings for each individual output method (e.g. OpenSL ES, or Hi-Res) and each output device (e.g. wired headset, speaker, Bluetooth) by using the No DVC and No Headroom Gain controls found via the Settings icon next to each option in PA Settings -> Audio -> Output -> (method) -> (device).
You can check the current status of DVC while music is playing by using PA Settings -> Audio -> Audio Info (or long-press on the metadata line at the bottom of the Player screen). The status is shown in the 'DSP' section. You can also see it noted on the Equalizer screen.
Note: If you are using the main Poweramp app and the Poweramp Equalizer app at the same time (which is not generally recommended anyway) then you must only enable DVC in one app or the other, not both.
Bluetooth Absolute Volume
When using Bluetooth, there is a facility for the playback device (e.g. phone) and output device (e.g. headset) to automatically synchronise their levels. This feature does not work nicely with DVC and can introduce lower or fluctuating levels, or distortion, so you should turn one or the other off.
There is a control in Poweramp which will disable DVC automatically if Bluetooth Absolute Volume is detected: PA Settings -> Audio -> DVC -> No DVC for Bluetooth Absolute Volume. However this is not always a reliable process so it is generally a much better idea to simply disable that Android feature completely which can be done by tapping Switch Off Bluetooth Absolute Volume in PA Settings -> Audio -> DVC, or by going directly into your Android Settings Developer Options and turning on the switch marked Disable Absolute Volume.
Samsung Hi-Res 384kHz
DVC is not supported for Samsung Hi-Res Output set to 384kHz. Select 192kHz to get DVC and hi-resolution audio.