I'm thinkin' a heavy throw rug and decouple the speakers from the floor. Air would be the quietest for the neighbor. Anything touching the floor you're gonna have to be careful. Springs are dirt cheap, pods there are some better than others and air ride is air ride. Rubber anything, there is still bass coupling.. I sure like the way bellowed pods work for the listener. The neighbors I always went with air..
We went from waves in his pool to a slight 1/4" foam. :-)
Regards