Dell Precision 470 Power Problem

First post so hi all :)

I've inherited a broken Dell Precision 470 Workstation. The  symptoms are as follows:

• Won't power up (5V standby power only)
• Green power LED on front panel
• Green STR_LED on motherboard
• Yellow AUX_PWR LED on motherboard
• No beep or VGA activity (no BIOS screen)
• ABCD diagnostic lights all off

I have verified the PSU is ok by shorting the PSON# signal to GND. That causes the board to power up while shorting the connection (fans spin up to full speed) but the minute I remove the GND connection it shuts down again. If I remove the Front Panel IO board and jumpstart the PC it stays on but there is still no BIOS or VGA. My conclusion from this is that the Front Panel is holding the motherboard off for some reason and that the fault lies either on the motherboard or with faulty CPUs. There were two 3.4GHz Xeons on the board. I have removed CPU1 and the associated VRM and still no boot. Next step is to try removing CPU 0 and replacing it with CPU 1 to see if it makes any difference. There is no evidence of damage to the motherboard (no exploded capacitors etc).

Any guru suggestions on what might be the issue? What errors will cause the Front Panel board to hold the motherboard off? I am reluctant to throw money at a new motherboard if it can be avoided. Am I right to assume that, if I remove the front panel IO board and jumpstart the PC by shorting the PSON# to GND, the board should boot ok (i.e. the front panel board is not required for it to boot).

Thanks in advance :D!