B&W Matrix 801 Series II speaker spikes

Those are nice speakers. I seem to remember that whenever I saw them for sale they would have Sound Anchor stands available. You might check to see if they are still in business. I’m sure you can find them for sale somewhere online.

packeted

Before doing anything, what type of floor are they going on?
Cement Slab
Suspended wooden floor
Suspended Cement sheet floor
Suspended Ply floor
As if you "couple" a speaker to the last three with spikes, you will do no favours for your bass, as the floor will become a "sound board" for the bass, and ruin it.
For this type of suspended floor you need to "de-couple" the speaker from the floor so NO energy is transferred into it. Sorbothane is your friend for these types of suspended floors. pick something that will take the weight from ebay, or pay a fortune for and audiophile product. 
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR0.TRC0.A0.H0.TRS5.TSS5&a...

Only the cement slab should have the speaker spiked into it.

Cheers George

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 :)