How about a Rockford Fosgate 3Sixty.2 or a JL audio Clean sweep. By using eiter one of those he can use the stock head unit and still upgrade the speakers and will also be able to add amps. This is te way I went in my FX.
Infiniti FX Owners Forum (The Org) • View topic - Pics of system install in TommyGuns FX
Check it out. I think you will like it!