Steve,
There has been a number of threads where guys have mentioned their machine will shake, rattle, & roll at higher rapids so they have to run slower. I followed the 'more mass is better' theory and I'm glad I did.
My ZX45 is capable of rapids up to 350ipm and even at that speed with two 6" vises on the table there is no discernible shaking or movement from the stand. The machine is sitting on 3" of Corian (Solid Surface Mat'l) that is glued into a solid block. Threaded rods pass through the Corian 'riser' and bolt the machine solidly to the almost 300lb stand. (3X3X.25 box tube completely filled with sand)
Casters with built in leveling pads make it moveable around the shop when necessary, yet isolate the machine from the concrete when leveled so it's quiet & solid as a rock!
'Bolton ZX45 Conversion' is my thread on the Zone if you want to look
gd.marsh