Viper4android is actually a universal sound mod
So if it is activated any sound coming out of the phone will be altered by it , be it music or youtube video or anything else for that matter..
But since poweramp has its own equalizer you'll have to disable that and select viper4a in MusicFX in Tone/vol section
Btw which rom are you on? Because it worked for me on stock and cloudyflex but not on mahdi (viper4a doesnt affect poweramp)