Blu-Tac is excellent for attaching speakers to stands.
Despite being squidgy when formed into shape in your fingers, it is hard enough in place to allow vibration to pass through to the support where it can be dealt with by support damping (sand etc) and spikes.
It also prevents accidental dislodging of the speakers.
It has a small degree of non-Newtonian property as a material. (Like Birds custard powder mixed with a little milk or silly putty.) Soft to form and spread but hard to impact/vibration.
Despite being very soft stuff it is very secure. (Make sure to twist speakers off stands when fixed on with Blu-Tac. Pulling them off can rip veneer off if not careful!)
[Edit] Despite the above, do NOT attach speakers with custard powder. It would be a trifle messy!