Umbucker,
This is a fairly normal occurance, especially with downward firing subwoofers!
I would suggest that you consider a granite plinth. You may use one on top of the sub to weigh it down and optionally, one below.
Besides weighing down your subwoofer, these audio isolation plinths also dampen any induced vibrations and help eliminate the 'smudging' of low frequencies transferring through the floor or carpet - resulting in an improved soundstage, tighter bass and a refined mid-range.
I use these myself for both my subs and the difference is certainly clearly audible to me.
Ronald