Just go on Crutchfields website and look it up. It's going to tell you that you need the one with the amp bypass kit so you have to decide whether or not you want to continue using the stock subwoofer or not. If not, you can just use a regular harness.
You really don't need to do the bypass. I simply didn't install the bypass harness. Just use the reglular harness and that will run your speakers. Most head units have enough power to run the 4 speakers effectively but at some point you might want to invest in a 4 channel amp.
Someheadunits put out more power than the factory amp so in some cases no it wouldn't sound better through the stock amp. Like Eric said, eventually you may want to invest in a 4-channel amp. I know I am eventually.
This is the problem most all kits say Mach,Jbl ,Premium but no audiophile. I did Email SoundDomain and they replied that it would be best for me just go get it locally do to the fact that there is not one that lists it, plus I also have the DVD system in it. Everything is getting ripped out and put in the old storage. I had started buying stuff for my Escape but it got totalled the same weekend it was to be installed
You need to call Crutchfield. They have the audio system you have in your ride. You have the MACH system in your X. So tell them you have the MACH system with the DVD system. They will tell you the exact harness you need.