Although speakers are rated for a certain impedance (i.e.- 4 or 8 Ohms), the actual impedance curve may vary with frequency (speakers have inductance). To compensate for non-linearity of speakers (on mainly subwoofers), Zobel circuits are used.
An L-pad circuit attenuates 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, they don't induce any phase shifts, or affect frequency response.