I highly recommend the Class A/B P series models. I have a P400.4s amp (100X4 or 400X2) that ran my entire system for over a year. It powered Infinity Kappa Components in the doors (and they sounded wonderful) and very adequately powered a 12w7 in a ported enclosure. I have also used 2 P300.2s amps that sounded great and were very reliable, I was really upset when they were stolen from me.
I don't have any experience with their Class D amps, but I've heard they are good as well.
As far as I know, most of planet audios amps are the same or very similiar internally, however a few of the amp lines use higher end components such as the Big Bang and the Apolcalypse series. Don't hold me to that though as I may be wrong.