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

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. 

I have a Krell KSA 300S which I have built a crate for. This amp weighs 185 lbs which makes it particularly hard to handle. I have all the original packaging but I built a crate anyway. Here is my experience:

I built the crate so that I could take the amp to the repair shop (180 miles away) because I needed to get it recapped. I did this because my experience moving the amp upstairs into my house was so difficult that I didn't want to go through that again. It's also stupidly hard to get the box into the back of a pickup or SUV and get it out again. People who have never tried to pick one of these up can't imagine how heavy it is.

The problem with any type of cardboard box is that there is nothing to hang onto. It is really difficult, even wearing gloves, for two people to carry a 185 lb. cardboard box that size. All I could think about was, "Oh God, what if we drop this."

The key feature of my crate is that I mounted two handles on each side. These are regular handles that I got at Home Depot and I used hardware that is suitable for the job. The handles make all the difference in the world. Two people can now carry the crate much more comfortably.

I pack the amp in the inner Krell box with the foam pieces and then put the box in the crate (I don't use the outer box but I kept it).. You probably can get away without the original box if you build the crate to the right specifications and use proper padding. However, when you are designing the thing, think about how you will get the amp in and out of the box. Using the original inner box,  if I ever need to ship the amp I can screw a lid on the crate, lash it to a pallet, and it should be hard even for FedEx to break it.

I used 5/8 plywood with interior angle brackets. I screwed and glued along all the edges and I put two coats of Varathane over the whole thing. It looks like it means business and is worthy of a $9000 amplifier.

I believe you can order packaging materials from Krell but I've heard that they are expensive. I would strongly suggest buying the packaging if they will sell it to you. Maybe I'm guilty of overkill but I've had my amp since new, it has been moved 4 times, and it is still in pristine condition. I'm really OCD about my gear and I would never forgive myself if I damaged this amp.

You will figure out the puzzle on how to pack the amp into the original carton and then put that into the box. You can't lift the cardboard box out of the crate so you have to gently flip it over, take off the crate and then unpack the amp.

Good luck. You are welcome to PM me if you want any detailed information.