Peanuts, air pillows and bubble wrap won't cut it. If your amp or receiver can move around in the box even an inch, that's bad. Wrapping plastic wrap around pool noodles is a great option. You can buy plastic wrap for shipping on a spool with a handle at Office Depot, Staple's and Wal Mart.
Cut the pool noodles to the length of each side, and secure tightly with wrap. It's ok to split a noodle for the sides if need be, but don't do this for the more delicate face and rear panel. Place the noodles on the front panel to protect delicate switches like
toggles. If gaps still exist, fill them with foam, or pieces of cardboard.
Another option is expanding foam. This will usually need to be done by a professional. Here is an example.
If any gaps remain in the box, use pieces of carboard for the sides. If there is space on top, cut the box down to size, and trim the flaps. If you are shipping to our shop, be sure to include the return label.