The best way to isolate my subs from the floor?

I live in an old home with wood floors that seem to be a little weak. My room vibrates whenever my subs kick in (I have two for loading). They are Von Schweikert SR1's, so these are musical subs, not at all boomy, and they are turned down. It's really an issue of room construction.

Do the products available for other types of speaker isolation work with subs? They are downward firing, so I worry about what I put under them. Can I screw up the sound by putting them on the wrong material.

BTW, on the matter of money, I've spent most of what I had to spend this year, but this can't wait. Cheaper options appreciated. :)

Joy Elyse

p.s. I rent, so nothing permanent and nothing that might damage the house.