I am in the process of redoing my setup, car had mach460, head unit is a pioneer and it has monster wires adapters to use factory amp
Right now I have infinity 6.5" 2-ways in the doors running off the factory amp which sound just fine, the weak link was the OEM factory mach tweeters, once had the outside edge deteriorating away, I replaced with a set of aftermarket tweeters and honestly, it sounded worse then if I had just unplugged them altogether
I ordered a set of infinity 6x8 components, going to wire these up in the front using the factory amp (for now, more info to follow) and move the infinity 6.5"s to the rear replacing the factory speakers there
This is just my temporary setup, soon to follow will be a 4-ch amplifier running all speakers (completely bypass OEM mach setup) and possibly an Infinity Basslink (vert has a small trunk as it is, dont want to loose anymore space to a sub box)