One possibility is that the tile helps keep the floor itself from being energized. Wood floors can serve as a giant subwoofer soundboard.

Of course if you have a concrete floor this theory is toast.

My experience was just the opposite. I wasn't getting well defined bass with my newer Wharfedale SW-300 downfiring 12" (and much less energy on movie soundtracks than my front fire 15"). Wharfedale recommends a slab in the case of thick carpet, so I got an 18"X 18" X 1/2" thick square of polished granite for the 12 incher and switched to the rubber feet. With the built in remote controlled active crossover/volume I can tighten up the bass on multi-channel music. Charlie Hayden, Brian Bromberg, Talking Heads and other bass heavy discs are much more tuneful with the 12". It made a big difference (good), but the 15 incher (on carpet spikes- no slab) conveys more dramatic, if less accurate, bass on movies, if you're into that sort of thing. Now I have to try to make room for both so I can switch between them depending on a/v source.