The actual class of amplifier (AB vs D) is probably not that important for you. Yes, they are fundamentally different different approaches to amplification, but there are a lot of other parameters that are equally, if not more important. Class D is generally more efficient than class AB, which basically means for a fixed sound level, the amplifier will consume less power. That is probably the main if not only advantage to class D over AB. And I'm not sure the power savings would make any difference for you. It's my understanding that class AB might genereally sound better because it is linear, whereas class D first converts the input signal to a digital representation, amplifies the digitized signal, then converts back to analog by filtering. So it is a very nonlinear approach, there's some chance that distortion or artifacts will be introduced, but I would trust most manufacturers have done a good enough job to reduce that so that you can't tell. Long story short, I wouldn't get one or the other just because one's a class AB and the other's a class D. All things being equal, I'd get the class AB to save money.
I'm assuming you're comparing the PDX-4.100 to the MRP-F600. Compare the frequency response data, SNR, Power specs. The MRP-F600 is rated better in some of these categories.
