For loudspeaker design and prototyping you'd want a simple, flexible system to play' with the crossover. You would want a simple way to vary cross-over frequencies, filter characteristics and levels. Cheap and flexible DSP solutions are available for the final implementation, but for prototyping it would be better to just be able to turn some controls and switches and listen to the changes in real time rather than input changes in a software application and then listen to the incremental changes. RR as he prefers to be called developed exactly such a flexible analog cross-over that's a great tool for the speaker designer, allowing continuously-varying cross-over frequency, on-the-fly transfer function change and custom-implementation of your choice of transfer functions. The author can also supply PCBs and a professional-looking front panel to help building this versatile unit.
A Universal Continuous-Time Active Filter