Hi
Interesting that. I have an A4000T here with the exact same problem, which I had put down to processor burnt out. But if you have swapped those over then maybe there is hope for us all. You say no access to the floppy drive.. I always thought that to be strange. Probably cus the machine is not getting past the initial check procedure. What you have is power activity but no computer activity. I put this down to a processor lock out. Maybe its another chip problem and could be the ROM. Dunno.
If all was fine one minute and not the next and the computer hadn`t suffered any trauma ( ie moving and dropping it ) it really points to an electrical failure. The trouble is finding it.
One last thing.... I assume that your other processor cards work .
No help, but hey, your not alone.
scuzz