Retired now, but worked in the A/V industry for 20 years, and it is possible to ship 60lb amps with nothing but bubble wrap and peanuts in a single box. I did this many times, and never had a problem. You must use the heavy duty bubble wrap and the type peanuts that are difficult to compress. What is most important is that packing is extremely tight with absolutely NO internal movement when the box is shaken as hard as you can. Even the slightest amount will increase as the package travels due to shifting and damage to the actual packing. Enough movement will eventually cause the damage to the equipment with no evidence to the outside of the box.
I'm guessing the amp was a little loose inside the inner box, then increasing as it traveled causing the damage.
I do agree that it is better, safer, and more foolproof to use the methods stated by the other posters.