Contrary to popular opinion, the decibel is not exactly a pure measurement. It is a multiple - amplifying a sound by two decibels multiplies the frequency volume twice over, amplifying it by seven or more may blow your speakers, unless you have kickass speakers. On the other hand, putting the amplification to a negative number will reduce that frequency by the multiple.
Adding decibel amplification usually comes in the form of a number with a + for adding amplification or a - for reducing it.
Guy 1 - "Dude I just put up the speakers to around +15 dB!!!"
Owner of Speakers - "OH SHIT!"