My house is only ten years old, but when I spiked my subs the whole floor shook. Sorry, but their is no substitute for trial and error. Try a box of sand on top of the sub.
Yes, trial and error. So that the trials don't break the bank due to absurd overpricing, you might check out International Equipment Components (google it) for inexpensive generic feet. To isolate my VR4s, I got eight leveler feet, added 1/4 inch of cork and a layer of felt, and for under $20, voila.