I have a couple of concrete slabs and rest the spikes on the concrete. I then finely adjust the spikes and tighten the locknuts until its very stable on there. I like what the slabs do and it does tighten things up. It does isolate the speakers enough from a suspended wooden floor.
Its not a massive difference as they were never boomy to start with. I was thinking that wooden floorboards isnt going to add qualities and I am right. Im listening to more of the speaker in isolation and it pleases me. Its depends what speakers you have but for a few quid per slab its worth a try. You can always use the slabs to prop up your barbecue outside
I made some fabric covered frames for the slabs and it looks very good