How to make Nextbook as loud as possible when playing movies with headphones?

I can't hear movies very well when at the gym or on an airplane and am using headphones with my Nextbook tablet that runs Jelly Bean 4.1. (And even if I'm in a quiet room, the sound is not exactly deafening.) I have the MX Player as well as a couple of sound boosting apps running. Some sound boosters give a background noise (like a rush of air) that tells me they're doing something, but it doesn't seem to affect the sound of the movie itself. Danger to my ears, which I often read warnings about, is the last thing I need to worry about at this point. I need to boost the volume as much as possible on this thing. Thanks very much.