Once the main casing is removed, you'll need to remove the back panel. Keep the amplifier on its side and remove the two, Phillips #1 screws holding it on, on both sides of the amplifier.

You'll also need to turn the amplifier over, and remove 5 additional, Phillips #1 screws from the back of the amplifier.

Leave these screws in, as they aren't required to be removed, in order to take the back panel off.