Here's the meaning of damping factor and slew rate:
Slew Rate: This is a term used to describe how quickly the output of an amplifier can track its input. Slew Rate is usually measured in V / usec. The higher the value (up to a point), the better the amp is at potentially reproducing the subtle nuances and dynamics associated with music reproduction.
Damping Factor: This is a quantity which defines how quickly the amplifier can stop a reproduced frequency such as a bass note. The higher the damping factor, the better the amp will control the woofer and help reduce overhang distortion (again to a point). The damping factor of an amplifier is mostly dependent on the output impedance of the power amplifier and the ability of the power supply which feeds the power amp.
THD in a car amp is not that important. There's too much background noise when the car's running to notice THD of 1%. In a home stereo, it's a different story.