I'm assuming that what you mean is that you have reversed the polarity of the high input on the back of your speaker in relation to the low input.
If the crossover point is low enough it may be that you are simply putting the bulk of the music into absolute phase.
Another possibility is that you have some room resonances at the crossover point and by reversing the polarity you are getting a dip in the frequency response at the crossover point that helps to limit the room resonance.
As an experiment have you tried leaving the highs reversed and then also reverse the lows so that they are both "reversed".
If the crossover point is low enough it may be that you are simply putting the bulk of the music into absolute phase.
Another possibility is that you have some room resonances at the crossover point and by reversing the polarity you are getting a dip in the frequency response at the crossover point that helps to limit the room resonance.
As an experiment have you tried leaving the highs reversed and then also reverse the lows so that they are both "reversed".