I can't answer your question but I wonder which video player you use? I use MX Player and when I increase the volume to the "maximum" recommended level (still far too quiet!) level I get a pop-up asking me to confirm I want to exceed this level, allowing it to go a lot louder.
The actual maximum is louder than I normally need using my preferred portable open-back headphones (Sennheiser PX100-II) unless the ambient noise is extremely high, in which case I use my in-ear phones (also Sennheiser) which sound even louder and offer a lot of isolation from the surrounding noise - any louder gets quite uncomfortable.
The same pop-up appears with the stock Google music player so I assumed it's an android thing that works with any app but I don't know for sure.