bass affected by stand filler level?

Not a good "opposite proportion" at all. The less filler, the more open area you have to resonate and the thin walls do not have attached mass and turn the area into a resonator. REMEMBER: thin & light material + rigid + vibration = a classic transducer. Just research NXT for more info on this. You cannot "tune" a speaker stand. It either creates undesired resonance due to lack of mass... or it is silent. The only way it could sonically "help" to have a hollow or partially hollow tube underneath a BIG vibrating mass (called a speaker) if is you actually ENJOY distortion and a steel tube trying to reproduce frequencies... but there is no in-between. I personally do not want a steel tube or other non-damped material to attempt to "reproduce" frequencies other than my loudspeaker :) You will get lower bass response by moving the vibrations to the floor, where the sheer size is able to help reproduce low end extension. Worst case is the floor won't resonate and you will just get the actual loudspeaker reproduction response... both a win-win.

Audiavreseller, sounds good in theory, but all floors do resonate. If you live on a concrete slab as is typical construction here, your speakers will be shaken by the trains going by. Mainly I find that concentrating the speakers mass on a fine point sounds best, but with my Beauhorns, their gel base improved the sound. On my Acapella LaCampanells, I am awaiting the new Townsend speaker craddles which decouple the speakers. I am told they do magic.

A heavy braced speaker cabinet will mean it matters least. Dynaudio are pretty good. The box should not flap around as it has a fair weight for its size. So it is probably damped quite well.

If you had Harbeth's this would be a much more important issue....what works best is a matter of choice. Many people like the warmth of a flappy box...so this is not a crticism. Dynaudio however tend to make precision type designs.

That is a really good point Tbg... If the environment has vibrations from outside sources, it could transfer more to the loudspeaker. This just keeps pointing to an alternating material decouple as the best scenario with environment in effect. Solid mass to prevent too little mass and turning to a transducer and decoupling to remove cross-induced vibration. Shadorne is also right; in that the better the enclosure, the more ability it has to contain the resonance it cannot control from passing into the stand to begin with.