class D just means what kind of amp. It's how efficient it runs. Class D is more efficient than class A/B.. it will produce more power, less heat, with less electrical current. The disadvantage is that the sound quality won't be as great as the a/b amps. You don't really have to worry about these figures when it comes to powering subs though.
Anyways, i'd look into the JBL BP sub amps. They're reasonably priced, and will never let you down
