In the end, two of us carried the machine while two watched. I moved and prepared the stand myself. Can't speak to the weight differences between the 8400 & 9900. Doing it (the 9900) over again, I'd use only two people.
I have an 8300 that had to go down
a flight. Two strong people is doable on level floor but you will not get it upstairs with 2---remember, that's one guy on the bottom liftting most of the 300+ lbs. Don't know what the 9900 weighs but I will bet against you getting an 8400 upstairs with 2. Have fun