Place an appropiately sized slab of granite or stone underneath the sub. This can be kitchen countertop material - You can sometimes find off-cuts that a stone shop might sell inexpensively. That or a thick wood slab, like a butcher block, though I've not compared the two - I've only used the stone version which did work fine for me (I had wide-plank wood floors that were very old and tended to rattle otherwise).
Marco
Marco