Why Spikes?
I just spiked my Ascent-i's after an extended period of having the speakers on their flat screw-in feet (which make experimentation with placement easy). I have maple floors, so I stuck a piece of furniture felt under a coin, and the coin under the spike, to protect the floor from the spike. In Canada, a Loonie is ideal, and makes a great floor protector system for a total of $8.
Anyway, there is a reason why ML ships spikes with its speakers - they make a big difference in the ultimate performance of the speaker (on a suspended floor at least). While the bass is obviously tightened up, the real improvement from my perspective is how spiking clears up the midrange and top-end. Attacks are much clearer and faster, which makes imaging a lot tighter, and timbres even more realistic. The bass improvements really increase image depth as well. Concrete slabs etc. might increase these effects, but not if they are preventing you from just putting the spikes on your speakers and enjoying the music, so put the spikes on whatever coins you have next to your keys, and then finesse them (marble, concrete, etc) all you want later. You will be very glad that you did.