Okay, I do NOT know if this will work on SE-B, but it's the Generic Downgrader. You simply place the update EBOOT (in this case 1.50) in the correct folder, run the program, and viola!

This is a great alternative, as there are no firmware dumps required!

Again, I don't know if this will work. It should, but I wouldn't try it just yet, unless you're willing to take a risk.

Here's the link:
Dark_AleX's Update Flasher (Generic Downgrader)