Myself, I LOVE my Mach 460 combined with a descent head unit, it's all I'll ever need. My wife says she can hear it booming when I pull in sometimes. I just wonder what it would sound like if I replace the crappy factory speakers int he Mach 460 box with some good 5x7's.
I also love the Mach 460, it's a great sound system IF you fix its flaws. As you implied, the factory head unit holds the system back; put in a decent aftermarket head unit (with a Metco adapter), and the system wakes up. The only other thing I feel it needs is good sound insulation in the doors. That eliminates the muddy midrange and vibrations you get when the bass hits.
Careful replacing the speakers in the Mach 460. The main 5x7's are not only midrange but are also subwoofers (kinda). If you replace them with aftermarket, you lose all your bass. You'll definitely need to add in a subwoofer.
The Mach 460 is actually made by another company, licensed to Ford. Can't find their website right now, unfortunately.



