(This tips are not recommended for a subwoofer)
1)Place your speaker inside a ported enclosure. A speaker enclosure will help gain some extra volume, as it enhances the speaker's volume ability, making it perform better.
2)Place the speaker in a corner of your room.
3)Reduce the ohm impedance of the speaker. The lower the ohm impedance, the less electrical resistance, which will make your speaker sound more louder.
4)Remove anything that is in front of your speaker, as this can block sound waves.
Above are all the method's i used and they worked perfectly for me, still if you want to make your speaker extremely loud, you will need to buy a amplifier to make that happen.