rePhase is a FIR generation tool for building fully linear-phase active crossovers with arbitrary slopes.

It also let you manually compensate for the phase shifts of your loudspeakers and existing crossovers, be it active or passive.

Once generated, the FIR can be applied using any hardware (openDRC, miniSHARC, Najda, …) or software (foobar, convolver, JRiver, …), stereo or multi-way convolution engine.

sourceforge.net/projects/rephase

Additional resources:

Equalizer APO, REW and Rephase WOW! ~ diyAudio
en.wikipedia.org/wiki/Digital_room_correction