Flash the PSgrade dongle with the 3.50 downgrade flash - this will put the PS3 in service mode.
Put the 3.55 Lv2diag.self FILE 1 on the USB key and the 3.55 downgrade firmware (3.56 which is a 3.41 spoof)
Now your PS3 is running 3.41 but it thinks it is now 3.56. From here you can downgrade again to 3.41 modified and use the recovery menu to flash 3.41 OFW.
For the most part you cannot flash to a lower firmware then what it left the factory with. My slim had 3.40 on it. I tried a downgrade to 3.15 and it failed so I put the the 3.41 modified pup on the stick with Lv2diag.self FILE 1 and it got the ps3 going again.
best of luck.