According to this thread you aren't alone:
Poweramp hijacks the headset controls(wired & BT) from all my other media players... I use headset controls with the stock mp3 player, pandora, xiialive, & fm radio.. they all surrender the headset controls when not in use, but when I bring poweramp in the mix the headset controls only work with poweramp.. I went into the headset options in poweramp and disabled everything, that stops the controls in poweramp, BUT doesn't restore the headset controls to the other players when I want to use them...
but according to the last post by the dev this should fix it assuming you are running latest version (uninstalling would probably restore control as well):
Not exactly. I've made today a workaround I mentioned above - so unchecking Headset Buttons option in PowerAMP now should properly redirect next button presses to other players (applies to 2.2 only). This will be included in next minor release, coming beginning of the next week.
Thanks for the report again.