Wall mounting your deck to a load bearing structural wall would be your best option.
Think of your upper story floor to be a diaphragm attached at the edges of the load bearing walls.When the floor is disturbed, there will be more vertical movement at the unsupported center of the floor and less at the supported edges near the wall.
Footfalls,etc will thus cause a floor mounted stand to sway/rock(lateral) as well as bounce (vertical).The taller the stand,the more exaggerated the swaying/rocking motion will be.
Any turntable (and particularly suspended designs like your Gyrodeck) will react quite negatively to any lateral movement.Tho obviously having no motion at all is better, your Michell's long travel suspension is designed to deal with vertically oriented movement/vibrations fairly effectively.
Increasing the mass will only change the rate of the floor's motion but will not do anything to stabilize the flooring & stand.In addition,the increased mass will store more energy in the floor, resulting in a longer duration for the energy to dissipate.
A wall mounted support should provide you with the least vertical and lateral movement as well as storing the smallest amount of vibrational energies as you'd only be supporting the mass of the turntable and shelf.
If wall mounting is unacceptable or a monetary concern, you could move your present rack very close to a load bearing wall and improvise a bracket to solidly affix the stand to the wall(attach it near the top of the rack), thus mimicking a majority of a wall support's benefits.
You could alternately place the turntable on a low stand as close as possible to the wall to minimise the swaying effect. The inexpensive IKEA Lack sidetable(~$15) makes a very good performing low mass, cost effective dedicated low stand option.
Good luck.
Best,
Ken
GreaterRanges/Neuance
