How to physically move large heavy 100lb amplifiers easily (up and downstairs)

You never said how many stairs. If a straight shot (as we used to say in Chicago), You could buy 2-3 sheets of 3/8 plywood and lay them on the stairs (connected with "menders"). Just tie some cloths line around the box and sprinkle some talc on the chute to lessen friction. Just drag the sucker up the stairs (amp in box of course, two people better than one).

 Yeah, use the straps but invest in half a sheet of 3/4 inch plywood and make the "perfect platform" for the amp (trimming required).