If your going to look at your backside it's best to use a mirror or get a doctor to do it for you...
Nah... seriously though... way back about 87 my old A1000 developed the same fault (ie: blank white screen at boot up) at that time I didn't know a lot about it and took it back to the shop where I bought it...
It cost me nearly 150 quid to get it repaired (as it was just out of guarantee) but the guy who fixed it at the time told me it was the CPU that was dead and he had to replace it. I don't know to be honest if he was telling the truth as I never checked and was just happy it was working again...
I know you've said you have tried two CPUs but you never know they could both be faulty. Also I would check out all the socketed chips first and connectors as you never know something may have worked loose and just require re-seating before you go spending any more cash on it...
