Down-firing Sub makes floor rattle

I think I've got some bad news for you. The vibrations that are being transmitted to your floor are more likely than not airborne, even that may seem counter intuitive. A down and dirty check - put your hand on the sub when its making the floor 'rattle' and see if the sub itself is producing a lot of vibrations. If not you might get some better help by shoring up the floor. JMHO.

Newbee has good point. Ultimately everything will rattle. A plywood sheetboard somewhere could use some extra screws perhaps? A lose air duct mounting? A heavier carpet or underlay may help too. Also try moving furniture around or put you hand weight on different things (touch windows/doors) - it might not be just the floor.

thanks for the advice. i will try some more things...wouldn't front-firing the sub be better in my case?

Yeah, you're really not going to be able to solve this, other than balancing the sub out properly in the first place. Basiclally, down-firing, front-firing, slab, no slab, or even suspending the sub in mid air for that matter, isn't going to do anything about the airpressure and bassmodes in your room from being excited!!! Basically, unless you do something to your floor, it's going to respond accordingly. That's how bass works.
conversly, people think if they move their sub out from the walls, that it won't be as boomy in the room next to it. WRONG!!! That's not how it works.
Acoustics are what they are. You'd need to, like I say, just make sure you're sub is balanced out in level with your mains/the rest of the system (other than sub being way louder than the mains), and add some mass to your floor!
Good luck