Did you find and use BS.Player's Equalizer (more specifically: preamp) already?
If no; try that first. If skin doesn't show a button for equalizer:
(rightclick >) Audio > Equalizer
If yes; you can further boost using a Winamp2 DSP plugin.
Also AC3 filter can be used to increase volume.
(Let us know.)
P.S.: How to find audio decoder:
(rightclick >) Options > Filters > Advanced > ..audio ...