I had to deal to deal with the same issue. My Tidals don't have spikes but the footers need to couple to something solid. I ended up being very happy with squares of granite (see photos in my system description.) The bass tightened up very nicely compared to lighter weight limestone tiles I'd used before that. The polished surface of the granite is a flatter plane than the surface of most concrete floors, no problem with uneven contact. As to worries about not being coupled to the floor compromising sound, one just has to try it and see I suppose. I don't see how the coupled mass of heavy speaker and stone slab is going to be making any movement of sufficient amplitude to diminish performance though. The experiment isn't particularly expensive, a stone supplier/kitchen remodeller only charged me $100 for two good sized slabs with polished edges. After replacing the carpet in our home with a floating strand bamboo floor, the speakers sound just as good on the floating floor as they did on stone slabs. We did put down a high quality acoustic foam sound transmission attenuating underlayment.
Anything you can put between spikes and carpet?
I've been playing around with my speaker positioning and a recurring issue that I've never quite been able to satisfactorily resolve came up again. While my speakers - Verity Fidelios - have pretty long adjustable spikes on heavy brass footers, I never feel like they're making especially good contact with the subfloor under the carpet and pad. I think I could possibly tighten up my bass response with a more solid contact. Is there anything that I could put the speakers on that would give them a more solid coupling for bass purposes? I was thinking of some heavy MDF squares, or even Corian, but then of course the board is not coupled to the floor. Any thoughts?