Just please please 'seal up' the amp itself in a plastic bag of some sort (big trash bags work nicely) so you aren't picking / compressed-air-blowing styrofoam bits out of the speaker cone and tube sockets for an hour! Styrofoam peanuts can be a real pain in this regard, but so can solid block styrofoam as little bits of it come off all over the place as well.
Also- keep the preamp tubes in, but pull the
power amp tubes, box 'em separately so they aren't smashing against eachother, then wrap them in bubble wrap. And if you can, DON'T put the power cord in the cavity in back of the amp. It can smack the preamp tubes and any exposed speakers in the back.
-Ben