Class D uses a digital switching power supply, so they do not have the heat or weight of a class A/B. Many subwoofer manufacturers have gone to digital, or Class D, amps because they can get huge amounts of power and still fit the amp inside the cabinet and keep it a reasonable size. There are many companies who have now over come the problems with digital amps, namely the distortion factor. T+A comes to mind, excellent sounding gear. To give you an idea, my LINN amp is actually class V but it's similar to D or T, it weighs 11 lbs and can deliver 5x125 wpc. I believe that is really conservative because it can rock 5 Dynaudio speakers as loud as I care to listen without a ripple. You can see how this type of amp could be valuable in an amplified speaker. I can't remember exactly how efficient Class D is but it's very high meaning it uses most of the power it draws, less waste, that's what's meant by "going green". Going green is very politically correct these days.

Class A/B is your typical design you see most. From what I've seen the class A/B is still the choice for brute force and driving a difficult load. It is much less efficient and much of the energy wasted is dissipated as heat.