I would crossover your sub naturally in the nighborhood of 50/60hz, where you hit the -6db point from your speakers. This will take care of the boominess. As for the volume level, you can use white noise and music and do it by ear.
Most of the music is played at above 50hz, so there's no need for sub above that unless you like fat low bass. To me, a sub is meant to be used for sub-bass (40hz and below).
Most of the music is played at above 50hz, so there's no need for sub above that unless you like fat low bass. To me, a sub is meant to be used for sub-bass (40hz and below).