hifiman5
Seeing you have a cement slab, this is the best. I would spike all speakers directly into it, mains and subs. As there’s no way your going to vibrate a cement slab. And doing this you’ll have absolutely no back and foreword or sideways movement of the speaker/s, and the imaging will be as good as it gets doing this. As any waisted movements of the speaker will take away from the imagining by the drivers making the speakers move ever so slightly.
Unfortunately on a suspended floor the last thing you want is to couple to the floor with a spike, as then the floor then becomes a sound board especially the bass as well. So decouple (no spikes into them) for these types of floors and try to make the speaker not rock as best you can.
I’ve found on a suspended floor the best was to put a thick oversized cement paving slab (paint it black or whatever for the wife factor) )on the carpet under the speaker and spike the speakers into it. This way the paving slab is still de-coupled from the floor by the carpet and you can still get the speakers quite steady on it.
And paving slabs are cheap only a couple of bucks each but you want them to be around 2" or more thick and larger than the speaker the speaker that's going on them.
Cheers George