THD - It's simply the unwanted noise introduced into the signal by the amp. Every amp injects SOME noise. Theoretically lower is better, but except for the cheapest of the cheap THD isn't going to be an audible factor.
S/N - again - you can measure it, but there's not an amp built today with a S/N ration bad enough that it's going to make a difference in a car.
DF - well, this might be the most useless of all because it's completely subjective. Typically, higher is considered better, but some $1,000+ tube amps have DF's in the single digits and are considered excellent SQ amps.
Here's a general guide. THD: lower is better S/N: Higher is better Damping Factor: Higher is better Slew Rate: Higher is better Channel Separation: Higher is better Frequency Response: Wider is better
If you want to study specs, try Zapco.com and Pyramid/Sony.