B&W Matrix 801 Series II speaker spikes

Thanks guys. So the floor they're going on to is a hardwood floor, suspended by a fir subfloor on the joists. They're currently on their casters on some mats to protect the floor finish.

I'm thinking of buying some granite/marble off-cut slabs from the place we got our kitchen countertop and making some plinths. 

I was thinking I could put felt under the slab, put that on the floor and then use spikes (and probably metal cups) to rest the speakers on top of the plinths. 

Any recommendations on this kind of set-up? Is this a good idea?

I used to have B&W N803s on granite plinths that were custom made and those had spikes that suspended the plinth above the carpeted floor.

Because you have a suspended floor that will drum because it will become a sound board if the speaker is coupled to it.
You need to de-couple the speaker from it.

Use some Sorbothane pads between the granite/marble off-cut slabs and your polished floor. Some have self adhesive backing you can stick to the slab.
https://www.ebay.com/sch/m.html?_ssn=dang-good-stuff&_from=R40&_trksid=p2499338.m570.l1312.R...

Then some Sorbothane D70 (firm) speaker isolation feet between the speaker and the granite/marble off-cut slabs. If you must use the spikes (there's no need to) they can go in the small centre hole.
https://www.ebay.com/itm/SORBOTHANE-MEGA-MOUNTS-SPEAKER-SUB-TURNTABLE-ANTI-VIBRATION-ISOLATION-FEET-...

Cheers George

Thanks, this is really helpful. Looking forward to getting this set up :)