I have note 7. I used to run this app on note 4 with great sound quality, headset or not. It used to high sound.
However since note 7, it just sounds horrible. I used Aux output and I had to increase volume to a very high output and sound started cracking by the time it became decently audible.
I have used mx player, and that app has Audio output higher than Poweramp at the moment.
I have tested this with both my phone speakers and Aux cable in my car.