Down-firing Sub makes floor rattle

An Aurelex foam platform should do the trick. This was very effective in taming the 160 yr. old sprung floors under my Vel DD-15.

http://www.auralex.com/sound_isolation_gramma/sound_isolation_gramma.asp

Turn the gain down.

And that's exactly the reason why for music one wants to get a front-firing sub with side radiators, something like Totem Thunder, for example.

Alex, when I was shopping for subs, Velodyne Tech Support told me that, assuming proper physical decoupling (slab, spikes, platform, etc.) due to long wavelengths there is no difference in room vibration caused by downward vs. forward radiator.

There's alot of great feedback above from everyone.

As an additional option, you might wish to consider grabbing a single Adona AV45AS amp stand; they work amazingly well as a subwoofer isolation and elevation platform. Depending upon whether you buy 6", 12" or 18" uprights (legs) and where you lock the x-frame into the uprights, you have a large degree of control over the separation of the sub and it's down-firing port from the floor.

The port then would be firing at a granite surfaced platform which would allow the wave to have minimal interference as it travels out of the port and then the many feet out in your room where sub-bass waves actually begin to materialize.

I have my REL Britannia B1 on an AV45AS with amazingly clear results (IMHO). Send me an email privately and I can share pics and other info with you if you like. Paul at Adona is also a great guy to deal with and has a vast degree of knowledge that he loves to share.

Check out http://www.adonacorporation.com/av45as.html

At $300 US their reasonably priced, look great and deliver real sonic benefit; I use them throughout my system. They're also modular so you can start small and over time, if you like, grow your setup.