Herbies Pucks floor spike protectors...

Several good suggestions made by all above. Take a look at all the products for speakers made by IsoAcoustic's in order to get a feel for other ideas. 

 @jtcf bravo !!!! add a nut and you have locking adjustable ! 

I use Herbie's gliders under my Wilson MAXX speakers to be able to move them on a hardwood floor. The use of the gliders did not change the sound at all. If I place my hand on the bottom of the speaker when played loud, I don't feel any vibration whatsoever due to the cabinet material and their weight, so that may be the reason they did not change sound. I really like them and recommend you give them a try; my guess is you won't send them back.

I didn't know Vandersteen made spike shoes.

A quick Google shows they should probably work as good as my Herbie's gliders, though some may disagree.
Some of this stuff can get a bit anal.
B

I use Herbie’s titanium Cone/Spike Puckies on three sets of speakers. They add a noticeable improvement to the sound quality. Tighter bass with better micro-dynamics.