As an engineer by training, I chose to follow the tradition of filling machinery bases with polymer concrete as it makes good sense to me from a strength and vibration dampening point of view. I do not have test results as my mill was never used in its original "as delivered" state. I immediately dissembled the whole mill for inspection, cnc conversion, and polymer concrete treatment. The best discussion on polymer concrete for mill bases and towers is given in the CNC Cookbook:

CNCCookbook: Epoxy Granite Fill