class D's have inherently more inaudible distortion (you'll note most are rated at around 1% at full power while a class a/b will be .0something). Doesn't matter at subwoofer frequencies hence class D amps are always sub/mono amps. Class D's tend to be about 20% more efficient than class a/b so they run cooler and draw less current at the same wattage but sonically (if you could here it) a Class D would be a downstep which is probably why the Diamond is a class a/b.