Poweramp uses Direct Volume Control (DVC) by default on most devices. This produces much better audio output due to the extended dynamic range. DVC especially helps to get extra powerful bass and maximum possible loudness.
Still rarely some (custom) ROMs, while supporting DVC, can fail with it causing volume issues. Poweramp v3: DVC can rarely fail for some devices/ROMs and Hi-Res output, while Hi-Res output itself with DVC disabled will function properly.