In same vein, if you are pleased with your present loudspeakers, but wish that they provided a little more headroom or clarity at high volumes, then this doubling of loudspeakers might be the best way to go. Active crossovers are certainly nice, but not strictly necessary, as one choke and one capacitor are all that is needed to make a passive single order crossover. My recommendation is to select a crossover frequency that is the geometric mean of the lowest and highest frequencies produced by your loudspeakers. For example, 600 Hz for speakers that extend down to 20 Hz and up to 20 kHz; 1400 Hz for that only extend down to 100 Hz (mini-monitors).
    F_geometric = Ö(F_low x F_high)
I also recommend using the shunt arrangement rather than the series, as it provides better damping. (The shunt arrangement attenuates the stopband by shunting the driver with an ever decreasing impedance; where the series arrangement attenuates the stopband by presenting the driver with an ever increasing impedance.) Of course, with a passive crossover, none of the power increasing effects are bestowed. 

their high frequency limits, as a two to three octave overlap is recommended to make this filter work well. However, as the number of crossover points increase, the more likely it is that this gentle filter slope will provide sufficient protection. 
   On the other hand, if you are adding a powered subwoofer to an existing fullrange loudspeaker, the 1st order highpass filter is a good choice for the fullrange speaker and a 3rd order filter for the subwoofer. This combination seems to work well in terms of amplitude response. It also has a minimalist aspect that is gratifying: the subwoofer contributes only its low frequency heft and then quickly gets out of the way of the lower midrange; the satellite loudspeaker sees a very gentle highpass slope that allows it extend down into the deep bass as it very slowly falls off in volume with decrease frequency.

   Those using zero feedback tube amplifiers, will find a preexisting 1st order highpass filter in their power amplifiers. The coupling capacitor that connects to the grids of the output tubes defines a highpass filter that is usually set to some very low frequency, such as 1 to 5 Hz.    Decreasing the value of this capacitor raises the crossover frequency. By choosing the right value, we can forgo the need for an external crossover. The formula is an easy one:
    C = 159155/Fc/R,
Where Fc equals the desired crossover frequency, C equals the value of the coupling capacitor in µF, and R equals the value of the grid resistor. Truly minimalist. One trick I have wanted to try for some time now is using a satellite loudspeaker (a sealed box with a Q of 1) along with a 1st order filter, which comes with a Q of 0.707 (set to the Fo of the enclosure).

Choosing Crossover Slopes
   In theory, nothing beats a simple 1st order crossover. It results in a flat, time coherent frequency response.  Actual practice differs. The 1st order filter provides a shallow attenuation slope that is too gradual to protect fragile tweeters from brutal low frequencies and too gradual to keep woofers from working up into

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