I have the exact same MSI board and it happily unlocked my 720BE to a quad. What bios version are you currently using? My board shipped with v2.3 on it and I haven't bothered to flash it since v2.4 doesn't have any updates pertinent to my needs.
I really don't suggest flashing your BIOS unless it is necessary; flashing BIOS can be a risky operation if you screw up. That being said, from looking at BIOS downloads on MSI's site v2.3 contained updates for AMD core unlocking so if you're using a version prior to v2.3 I would suggest carefully going through the BIOS update/flashing procedure. Probably would be prudent to use MSI's M-Flash to make a backup of whatever BIOS is currently on your board beforehand.
Unless your BIOS version is earlier than v2.3 I don't think updating it will help. The extra cores on your 550BE might be bad. Try the following BIOS settings, using manual CPU Core Control to disable the second two cores individually to see if one of them is good or conversely if one is defective:
Unlock CPU Core: Enabled
Advanced Clock Calibration: Auto
CPU Core Control: Manual (this will let you Enable/Disable cores independently)
hi my bios version is v2.3
when i choose manual of cpu core control i only see
core 0 enabled
core 1 enabled
no core 2 and 3