Subwoofer and bare floor

I have discovered that I get better bass from my speakers (hybrid electrostats with integrated subwoofers) if I put a folded up towel under the down-firing base drivers. I have them on bare floors, normally.

Does anyone have an explanation for this? Is it common for subwoofers to work better on carpeting than bare floors?

In case anyone is wondering, I discovered this while moving the speakers around to try different placements: I put them on towels to slide them around. Usually I take the towels out before playing them, but this time I forgot. The difference in sound is really striking.