I've shipped tube amps successfully several times and always remove the tubes and pack them separately.
I agree with Audioquest4life that original boxes are good to have, because they prevent the amp from moving around inside the box but I would add a layer or two of bubble wrap for shock absorption, even if it means double-boxing and having the bubble wrap between the two boxes. The rigid foam that is common in manufacturer's boxes will protect from one impact but it gets crushed in the process. It doesn't really provide any absorption of vibration like bubble wrap or resilient foam.
I agree with Audioquest4life that original boxes are good to have, because they prevent the amp from moving around inside the box but I would add a layer or two of bubble wrap for shock absorption, even if it means double-boxing and having the bubble wrap between the two boxes. The rigid foam that is common in manufacturer's boxes will protect from one impact but it gets crushed in the process. It doesn't really provide any absorption of vibration like bubble wrap or resilient foam.