A Class "X" Amplifier is a Class "D" Amplifier with a digitally controlled power supply. It is this highly sophisticated power supply that gives Class "X" an edge in overall efficiency when compared to a Class "D" Amplifier.
the difference in efficiency is 10% making the efficiency about 90%...thats what makes ma audio such high power amps...but i'm sure SQ suffers greatly because i can't find ne thing on how it sounds haha...i myself bought an hk-802sx ($400 shipped on ebay) and it's going to power an 18" RE XXX...ive got a dsp, external crossover, epicenter, and line driver...i'm hoping that makes up for the SQ loss