You might see if you can find a guy to move it. They probably wouldn't charge much. I watched our mover move my safe and they have it down on how to handle heavy loads.
You can also look into one of those "walking stairs" dollies. I think you can rent them. The basically walk the load down the stairs under control.
I would be careful about sliding something that heavy down the stairs unless you have a lot of strong help!
I checked around and folks want $200+ to move it. I will have 4 maybe 5 guys plus the dolly. I will check to see if i can find the walking dolly.