Congrats on your soon to be received 845s!
Is this a new pair of U845s or are you updating the cores from an earlier/smaller Ultimate version?
Also, how tall will the speakers be as some 845 versions are slightly taller than 8Ft. which can be problematic installing them in an average height room?
As to damping the tubing, rapping the tubing with my knuckles on my U890s barely elicits any noise which quickly dies out so I didn’t feel it was worth the effort. The thought just occurred to me that instead of using sandbags on a string, as some have done, you could try using a few pool noodles inserted into the tubes. The noodles are available with a diameter of 2.75″ which should match the ID of the 3″ steel tubing. Although the pool noodles would not have the mass the sand would have, they would still help to improve the damping of the tube with little effort or expense, worth a try.