Go to a hardware store and get a few different size bolts---8/32, 1/4-20, 6mm, and 8mm are the most common spike sizes. Try screwing each into the threaded inserts to find the size spikes you need. Also, go to the Symposium Acoustics website for info on their Rollerblocks, an interesting way to provide high vertical coupling between speakers & subs and the floor they sit on, with high isolation from horizontal/lateral vibration. Most of the vibration traveling through flooring and trying to get into your speakers/subs moves through the floor material horizontally, like ripples across the surface of a pond. Isolating speakers and subs from that energy while firmly anchoring them to the floor in the vertical plane (to prevent unwanted movement of the speaker cabinet, and to drain vibration out of the speaker/sub enclosure---though there is some disagreement about the validity of that theory) is a highly effective technique. Read the Barry Diament (an audiophile quality recording engineer) Audio website for more info. Go to "Articles", then "Vibration control for better performance". Also worth reading is "Setting up your monitoring environment", written for engineers but equally applicable to audiophiles.
