I have seen a few different setups using similar models (the marine version and somebody has a x303 i believe) without any problems. The harness needs to be wired up in a particular way but that seems to be the only "gotcha" if you will.

This Thread has a good step by step and instruction on the harness and entire install. If you order from crutchfield the included documents also have special instructions for the crossfire.

This thread talks about the stock amp being 4 channel and how to wire it up to control the subs/fronts independent.

I will be installing my own X305 this weekend and will post up my results/process in This Thread. I Have a couple questions outstanding (most likely due to the length of the post ) but I have it all pretty much figured out.