I've shipped a few that came in for repairs. When I pack an amp, I expect it to bounce down a set of stairs at least once in shipping. Here's how I do it.
Vacuum out the inside of the cabinet and clean the power cable, foot pedal, and its cable. Wipe down the cabinet with a damp cloth and dry it well.
Remove all tubes and wrap each in at least two layers of bubble wrap, taping it securely.
Gather the power cable and place a zip tie on it. If it has an IEC connector on the chassis, unplug it. Gather up the foot pedal cable with a zip tie and wrap the pedal in a layer of bubble wrap, securing it with tape.
Lay the amp face down and place several layers of bubble wrap over the speaker. Tape all the individually-wrapped tubes into a bundle and place them into the cabinet. Place the power cable in there, as well as the amp's foot pedal. Dump a few handfuls of packing peanuts into a large trash bag and place this inside the rear of the cabinet. Press the bag from the inside down around the speaker and the tubes bundle, and continue to fill with peanuts. The idea here is to fill the entire cab with peanuts. The bag keeps them contained, rather than having them loose.
Cut a square of 1/4" ply or several layers of corrugated cardboard that fits over the speaker grill. Wrap the entire amp in a layer of packing paper then tape the protective panel in place. Place the amp in a large trash bag and tape it snugly around the cab; this protects it from moisture.
Wrap the entire amp in several layers of large bubble wrap. If you have foam shipping corners, place on the amp's corners and tape them in place before applying the bubble wrap.
Pack it in a tight-fitting carton...double layer is better than a single layer. Use fiber-reinforced tape--not paper or thin plastic package tape. Amps are large and heavy in their carton and will get pushed, pulled, and dragged around in shipping. You want to give it a fighting chance of arriving in good shape.
