Thanks for all the input, fellas. Much appreciated; with regard to my comments about switching my sub from the 350 to an SVS or HSU, it's not that I wanted to upgrade subs thinking this was going to solve the rattling problem, I was merely pointing out that I was getting ready, finally, to consider dumping my POS 350 -- of course, I am aware of the fact that by swapping in a new, more powerful sub like an SVS or HSU the rattles won't disappear and will most likely be progressively more aggressive...
I will consider the suggestions regarding adding treatments to the source of the rattles, which I believe is a metal decoration we purchased at Home Goods that sits above the fireplace, but I think I would want to go the "sub on an absorbing platform" route first. Any other input on these platform kind of devices? Are they worth buying?