Do you use spikes for tile/marbel floor?

If you have the option flip your spikes over and put the rounded end on the floor. This has worked well on my pergo and slate floors.

What about using spikes on a regular wall to wall carpeted cement floor? That's what I do. Is this a good thing? I have heard that it is and that it isn't.

Elb I never heard any objections to that approach; it makes sense to me. I believe that should work just fine; you're basically piercing the carpeting with the spikes & all the weight atop of them is bearing directly onto concrete.
I wonder who says that this won't work & what was the reasoning? I'd tend to disagree, although maybe I'm missing something?

I'd recommend a faily inexpensive tweak. You can use cones under the speakers that you can buy through Audiolabhawaii.com. These cones increased the dynamics and bass of my speakers. I've used many different cones and these are the best.