this is actually the parent website as far as i know.
i know you took mine and some other's advice to make the foundation of that machine much thicker, but from where you're at now i think it would be good for you to hollow out from the top down all the useless material internal to the structure of the vertical axis.
The very deep channel between the vertical axis rails has effectively reduced the vertical axis to two independent columns. i would suggest you hollow out those columns from the top down, a tapered cylinder aka cone or a 4 sided pyramid. the taper is optimal for both the mold required for casting and for moment of inertia considerations.
i've lost track of how big this machine is but i would imagine you could remove about 100 pounds of material from the epoxy granite casting. this weight reduction will significantly increase the resonant frequency of the base structure without decreasing stiffness significantly.
if you were to add that material back to the backside of the machine and make the web that connects the two vertical rails together thicker, you would then increase torsional rigidity and this might be worthwhile.
Attachment 400233
basically a cone projected into the structure as so. all of that can be removed with no impact to rigidity at all.