Depends on the woofer and the type of floor. I use Auralex Gramma (or SubDude) between my sub and the wood floor. It reduces the floor resonances.
Each speaker has two 10" bass drivers in the same enclosure, one driving down, the other driving forward. Each has it's own internal amp.
The two drivers clearly must interact with one another, and on the bare floor it seems the forward-driver doesn't produce much sound. With towels underneath the bottom-firing units, there is a lot more sound coming from the forward-firing units. I wouldn't say the bass is softer, or more defined, it is not as subtle as that; I would say rather its more an issue of there being more bass or much less bass. I do care about quality, but right now I'm facing a much larger order of difference: if I can't even hear the bass note it's hard to tell how well-defined it is ;-)
Anyway, thanks for comments!
Kal: thanks for your response, I did indeed google Auralex, thanks for reference. I wish my problem were as simple as a resonance, which would manifest itself, perhaps, as a booming or suckout in certain frequency bands. My problem is more across-the-board, covering a broad swath of bass frequencies. This is a hybrid speaker design where the electrostat panels really depends on support from the woofer/bass subsystem, which it is not getting unless I put the towels underneath.
Note that I have been talking to the manufacturer of the speakers and I believe my issue is... unique and not something pervasive. But we don't know what is going on, it's apparently very odd behavior, and any ideas anyone might have would be greatly appreciated. One theory is that the two bass drivers in each cabinet have been internally wired out-of-phase, but given extensive factory quality assurance, this is seems highly unlikely.
You can test for a polarity error with a bassy mono source by trying it with normal wiring and with one speaker reversed. The correct polarity will give a summation rather than a cancellation of the bass.
Floor resonances are usually wide-band since the surface consists of many planks with different characteristics loosely coupled to each other.
I cannot think of how the towel is having an effect unless it is damping higher frequencies that should not be coming out of the woofer.