Like spikes on large floor standing speakers, the idea is to decouple the speakers from floor or stands. So any "soft" materials will have absorbing effect and slightly smear the mid and bass definition, if you can hear it.
Hard material is better, but they will scratch the nice finish of speakers. I will recommend BDR cones which has rounded tip and will not damage the surface. Problem is they are slippery and speakers can "fall off" from the cones. That has happened to mine, cost a lot more to repair the speakers when damage is on top than some minor dents or scratches on the bottom of speakers.
Hard material is better, but they will scratch the nice finish of speakers. I will recommend BDR cones which has rounded tip and will not damage the surface. Problem is they are slippery and speakers can "fall off" from the cones. That has happened to mine, cost a lot more to repair the speakers when damage is on top than some minor dents or scratches on the bottom of speakers.