I agree completely with Bruce on this. The real reason I finally got off my ass and filled mine is because my very sharp-eared, engineering-oriented audio buddy (who lurks around here on occasion) heard something 'ringing' after big bass attacks (a tympani I think) and, after walking around the room clapping, identified the stands as the culprit. That was good enough for me. Clapping loudly at the listening position resulted in an audible ring from the stands. (which, by the way, are extremely rigid Lovan References with six columns. Each column is attached to the top and bottom plate by a very sturdy bolt, which my dealer apparently hired a gorilla to tighten.)
As I've mentioned somewhere else around here, I did notice an apparent reduction in bass after filling with sand (plus the kitty litter I couldn't get out). After some listening, though, the bass was cleaner than before, and had better impact--faster attack, faster decay. Logically, all the undamped stands could do to 'increase bass' is add noise at their various resonant frequencies. [Someone tell me if I'm wrong.]
More bass as a result of room interactions or sympathetic vibrations can sometimes sound like the warm, generous bass response you hear in some halls. This can seem like in increase in realism to the extent that many recordings (particularly commerical labels) do not properly capture this feature of the sound of a live performance.
But to the extent you're chasing after 'accuracy' rather than 'listenability' or whatever, bass that's not on the recording shouldn't arrive at the listening position. Of course, we all have severly imperfect rooms and there are lots of bass response problems; resonating speaker stands is just the easest one to fix.
I was interested to hear that you're using vibrapods. I have also experimented with them under the DC's. My initial impression was a bit of shock at the weirdly seductive midrange clarity and golden tone color they produced; the bass had a warmer, fluffier sound to it. After more listening, the bass seemed less controlled and less deep, even though further up in the range something very interesting indeed was happening. My current opinion of vibrapods is the same whether applied to speakers or components: they provide a measure of isolation, but their 'sound' contribution is ultimately defined by their material, and resonance properties of the vinyl (or whatever it is) tip frequncy response towards the midrange, slowing the bass and slightly attenuating the treble. The results in this regard were eerily similar using them (alone) under my tubed cd player and under the speakers. With speakers, however, there is another issue: the pods are not rigid, and as such they allow the driver, when it moves, to push the speaker back relative to the air, instead of vice versa. This obviously occurs on a small scale, but I think you'll find that pushing the speakers back and forth (without making them slide relative to the pods) will confirm this.
This is all theory, although it coincided with the results I wanted in my system (i.e. deeper, more controlled bass). Ultimately you have to use your ears and decide what sounds right. There is also the issue of whether you're voicing things for an optimized system you don't yet own (my current predicament) or for a musical result with current equipment. That's another rant entirely, and you'll probably thank me for not starting...
There may also be some acoustic magic tricks you can do to increase the apprent size of your room. My room started sounding bigger when I hung a persian rug on the back wall. The problem with that kind of measure is that it has a limited frequency response.
