You may be able to make music seem louder by using a player with a compressor or limiter plug in. the idea is that you reduce the volume of the loud parts and transients allowing the level of the quieter parts to be brought up and seem louder over all.
You'll need to know how it set it up properly and you'll likely make it sound horrible.
Another option is an equalizer, cut the bass parts that the speakers can't produce anyway but still take up amp power, and boost the mids (around 1kHz it's where the human ear is most sensitive). Again, it might not sound the best but if you do it right it can seem louder, or at least be able to cut through background noise easier.