The definitive software to listen to your crossover! You can create a 4 ways speaker (or WMTMW…), position the speakers, define the crossover and then move the listener to hear and see the response changing.

Multi-Sub Optimizer (MSO) is a free Windows-based software program for optimizing the bass response of audio and AV systems having multiple subwoofers. It minimizes the seat-to-seat frequency response variation in the bass region while simultaneously optimizing the response flatness at the main listening position. You can choose to have MSO perform this optimization on just the subs, or to include the main speakers in the optimization. In the latter case, it automates the process of integrating the subs with the main speakers, including delay / distance settings and EQ in the crossover region.
Although it’s possible to use it for identical equalization (EQ) of all subs at once, it does not depend on a global EQ approach like commercial room correction systems do. Such systems cannot reduce the seat-to-seat variation of the bass response. Instead, MSO performs optimization of EQ, gain and delay parameters individually for each subwoofer. This individual EQ optimization for each sub causes the variation of the bass response from seat to seat to be reduced, and the bass response at each listening position to be flattened.
MSO can be used to perform tasks that include:
foo_dsp_xover is a software digital crossover implementation that I have been working on for a while in my spare time as a hobby. It is a Foobar2000 audio player plugin that implements this digital crossover to achieve active multi-amplification.
This software needs to run on a WinXP PC equipped with a multi-channel sound card (e.g. 5.1 or 7.1 surround sound card) feeding the various voices of a multi-amplifier and loudspeaker active configuration set-up.
Pulseaudio Crossover Rack is a program to design and implement multi-way speaker crossovers using any linux powered computer with a multi-channel sound card and a running desktop environment which uses Pulseaudio as it’s sound backend. It also uses a set of LADSPA plugins, namely ladspa-t5-plugins for the heavy lifting of DSP/audio processing. It’s written in python3 and uses QT as the windowing toolkit.
The freeDSP is an open-source digital signal processor family for the do-it-yourself community. The applications range from active loudspeaker concepts (digital crossovers, bass enhancement, …) and room equalization over advanced musical effect processors to car audio signal processing.
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
Equalizer APO, REW and Rephase WOW! ~ diyAudio