Phase shift on a first-order crossover is 90 degrees.
Second Order (12db/octave) Two-Way Crossover
Linkwitz-Riley crossovers match attenuation slopes so that system response is flat at crossover point.
Butterworth crossovers yield to a peak at the crossover frequency.
Bessel crossovers have a frequency response between Linkwitz-Riley and Butterworth crossovers.
The phase shift on a second-order crossover is 180 degrees (reversed polarity).
Third Order (18db/octave) Two-Way Crossover
Phase shift on a third-order crossover is 270 degrees (-90 degrees).
Fourth order (24dB/octave) Two-Way Crossover
The phase shift on a fourth-order crossover is 360 degrees = 0 degrees (no phase shift).
Zobel Circuit (Impedance Stabilization)
Even though speakers are rated at a certain "resistance" (i.e. 4 Ohms), the actual impedance varies with frequency (speakers have inductance). To compensate for the non-linearity of speakers (on mainly subwoofers), Zobel circuits are used.
Re is the DC resistance of the woofer (can be measured with an ohmmeter)
Le (or Lces) is the electrical inductive equivalent of the driver.
L-pad (Speaker Attenuation)
An L-pad circuit will attenuate a speaker.
L-pads keep the load "seen" by the amplifier constant, affecting only the power delivered to the speaker. The power delivered by the amplifier remains constant.
Since L-pads are made from resistors, it does not induce any phase shifts, or affect frequency response.