Would I just stuff the oversized box with newspaper on all sides? is that effective?
Erm, blanket first, then a few inches of newspaper. That way it's a little easier on the amp. I'm going to assume it's a tube amp? If it is, would it be a good idea to send it separately?
Also, you should add a "This End Up" sticker, and a "FRAGILE" sticker, and a sticker with an umbrella, so it won't get wet... Or does the post office do that?