Down-firing Sub makes floor rattle

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.

I had the same arrangement with my mains and sub alignment and began at 0 degrees phase. After much testing and test relocating of the sub, I found the original position was the best (to remove a room mode or two) with 180 degrees yielding the best, most musical and appealing bass...My REL B1 is front-firing however it does have a floor-firing port.