Getting ready to install my new stereo....I have harness from Crutchfield that allows the use of the factory amp. I thought it was desirable to bypass the factory amp. when doing a four door speaker system? Is the HU amp superior to the factory (maybe a dumb question). Looks like I need a couple of Metra 70-1761 wiring harness'. I'm not sure Crutchfield will be much help on this install.