Try removing the driver board completely, then power on. This will let you know if there's something on the driver board mucking things up or not.
I just went through some problems on my firepower, and it ended up being a bad PIA (or two) on the driver board that was keeping the CPU from booting properly.
I'm starting to think that PIAs should've been named PITAs.
Good collection of Firepower info here (not sure why it's called "blackknightpinball" when it's really firepower):
specifically in the margins here:
Helped me out a LOT.