you need to protect all 4 sides of the amp better. get more bubble wrap, roll it up like the center roll of a paper towel roll (x4). Then tape that roll to each sides of the amp, so it has cushion. dont leave any airspace in the box. Fill the box with either packing peanuts, or more bubble wrap, so its nice and tight and wont go anywhere. Last, after the box is taped all up, write "fragile, glass" all over it, and draw arrows on it, so it sits flat and not on any of its ends.
you have to do as much as possible to ensure safe shipping, because the delivery a$$holes dont give a crap and throw our sh!t all over the place.