FWIW, without root you can use mxplayer for movies ( https://play.google.com/store/apps/d...videoplayer.ad
In the app, under settings > audio > volume boost, it will let you go 200%, which is nice to have the option. Just be careful because it is possible to damage your speakers. When I use it, I generally only go no more than about 150% to 175% because that's all I need at times and I don't want to overdrive the speakers. The app volume goes by 1 to 15 without boosting, with boosting it is 1 to 30.
Otherwise, I do a partial boost with mxplayer when using my bluetooth speakers (soundfreaq soundkick) and it makes a nice "mini-theater".
Hope that helps.