Do you have to move it around any corners? If not, since the stairs are carpeted why couldn't you strap it to a board and have some strong men very gently slide it down the stairs and onto 2-3 of those rolling things that movers use to move really heavy stuff and roll it to where it needs to go. Now lifting it to the top of the stand I'd suggest some jacks and hoist it a little higher than the stand and then slide it off the board you have strapped it to over to the stand?
Or call a piano mover and see if they would do it since they move pianos that easily weigh that much up and down stairs -- they must have a system to do that.