I am continuing to use kxproject exclusively in my vehicle. It just provides so much configuration that nothing else (even $500 studio cards) can really do. If the pop bothers you, then using the M4ATX's power delay isn't sufficient, I'd recommend using something like serial amp control that won't turn them on until your front-end is up and running.

I haven't implemented multi-zone audio yet, but I have used the multi-inputs to the dsp to properly decrease the volume of my media playback when the navigation voice is talking. (Speech is routed to one of the secondary outputs, which feeds the sidechain of a compressor to drop the volume of the front speakers completely, and drop the rear speakers about 50%).