If you want a small phase difference as possible, the slope has to be 6dB at the crossover frequency [Fx]
Fx [Hz]= Crossover-Frequency to calculate (3dB or 6dB Slope)
Slope= Slope at Rolloff-Frequency (fx)
R [Ohms]= Loudspeaker resistant at Rolloff-Frequency
Fx [Hz]: 700 Hz
Slope [dB]: -6 dB @ Fx
R [Ohms]: 8 Ohm
Order [dB]: fourth-order, 24 dB/Octave Crossover typ: low pass
http://mh-audio.nl/Calculators/CBBC.html
