B&W Matrix 801 Series II speaker spikes

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

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