Build a wooden crate or use a cardboard box for Amplifier.

I double-cardboard boxed my PrimaLuna integrated using using plenty of foam and bubble wrap. It successfully made it across the seas by freighter with all tubes plugged in and intact.

In my industry we receive heavy weight pumps in a reinforced double thick cardboard boxes. The pumps are typically bolted to a plywood bottom. This keeps them from moving around and getting damaged in freight. 

You could cut a plywood base to sit in the bottom of a heavy duty cardboard box. Set the amp in the middle and put a 2" piece of foam panel on top. Take some perforated strap and run it across the top and use shallow wood screws to pin it down to the plywood base plate. If you get it sufficiently tight and use a box with a few inches of play around the outside of your amp it should be sufficiently sturdy for transport. In my mind at least 

You need to consider how you expect the carrier to handle it. I recently shipped an amp, but it was only 28 lb. I built a crate and used a single metal gate handle for it to be carried suitcase style. After going from AZ to MI and back, I was amazed at it's condition on return. (FedEx) It looked like it only went around the block.

If it is going to suffer impacts, wood will transfer more of the impacts v. cardboard, so you need to cushion the inner carton well. But a Krell is probably heavy enough that it won't be getting tossed around. I would assume hand trucks are going to be used and build to make that easy as possible. If it has to ship flat, I'd assume vertical forces are of the biggest concern, so you would want sidewalls capable of supporting a top panel that prevents any kind of crushing. Maybe wood sidewalls and OSB top & bottom.

If it can ship vertical with the heavy end at the bottom, I would do a riser on the bottom ends that allows a hand truck to easily grab it like a pallet. I found it to be quite the engineering task, with many considerations to factor in. Obviously you want to keep size and weight in check, ensure maximum protection, build it so it can be easily unpacked and repacked and the easier you make it on the shipper, the better it will be for your equipment. Insure for the maximum and slather it with"this end up" and handle with care stickers.

Contact Krell and ak if they will sell you a packing box for your units. A lot of manufacturers will do this. I know McIntosh does. You would pay for the boxes but they are designed for your particular units. FWIW

Box them individually with dense foam and strap it to a pallet. Make sure you don’t stack anything on top.