to the op's original question, without going into technical details, class d amps are more efficient than class a/b amps. you apparently phrased the question incorrectly, as there are both single channel and multi channel amps that are either class a/b or class d. class d amps have become ever more popular thoughout the years for their efficiency. in the early days of class d amps, most were mono amps designed to run subs. today, many new ones are built for full range duty to power mids and highs. without inciting a debate over class a/b vs. class d arguments, i would definitely recommend a class d amp for subs. for the mids and highs, not as important, unless space is a concern, and you want the amp to take up less space. class a/b amps measure better on test equipment than class d amps, but most of those are inconsequential, as they are beyond audible limits of the human hearing.