1) Media player, I do not think one can get more than max

2) xmms, it has an equalizer, you can push ALL frequency to max, that would add 20db with some distortion

3) mplayer, do the following, it CAN bust your tablet speaker ,
http://talk.maemo.org/showthread.php?t=24349, use any editor you want, I found emelfm2 the easiest.
================================================== =
increase volume: add to the file /etc/mplayer/mplayer.conf
softvol=yes
softvol-max=500
================================================== ==
I have tried softvol-max=999 and I can hear my tablet across the hall
These are maemo.
KDE or alternative OS has players with equalizer can do similar to add 20db to max.

I recently bought this, it COMPLETELY shut off external noise and with a battery amp (adds 2.5db I guess), I can hear pretty good during my bus ride, yes, in a bus, not subway

Hope this helps,

bun