McIntosh Double Box?

Do NOT throw away the outer shipping box for any McIntosh gear.  It's part of the why McIntosh gear usually isn't damaged in shipment.  They spent a fortune figuring out the shipping containers, so use them.

It's an integral piece...

Do not throw away original boxes.  If you need to ship it back to the manufacturer for repair or any other service, they may demand use of that box.  Some companys charge a small fortune for the box.  McIntosh gear has glass faceplates that are easily damaged and this puts an additional premium on having original packaging.  I kept the original boxes for large Martin Logan Quest speakers by breaking them down; I taped them back to sort of their original shape when I shipped the speakers to their new owner.  

I'd pack the preamp in both original boxes to transport , that unit has tubes and vibration is a factor even if you have to rent a small trailer for other stuff and put the Mac in the truck or back floor of your vehicle on top of a couple of pillows.

Def NOT. It's double boxed for a reason and there are countless examples of broken faceplates with Mac product due to people cutting corners packaging a piece when shipping it. Additionally, if you do ship it in a single box and even if you insure it, if it gets damaged chances are high the insurance won't cover it as they want photos of the packaging etc, and they'll figure out it wasn't packed properly. 

If you're short of space for the cross country drive, then ship the double boxed C2700 rather than drive it.  Ground ship it and require a signature.