goofyfootCement slab floors are the best, but some like myself and you have hardwood suspended floors, but the floor itself will become a soundboard if the speaker is coupled to it in any way, really screwing with the bass.
Can I Live With A Hardwood Floor?
The best is to totally decouple the speaker from the floor, by ways of thick amounts of Sobothane type materials between speaker and floor to stop any bass energy from speaker being transmitted into the floor.
http://www.ebay.com.au/itm/Genuine-Sorbothane-4-x-64mm-Isolation-Pods-Domes-Feet-2-5in-Dia-LARGE-BIG-PODS-/370465792771?hash=item56417b6303:g:UA8AAOxy4dNS6cB0
It says 4 will take 34kg, which just happens to be your Quads weight, so I would use 6-8 per speaker so they don’t squash as much.
Also have a look in their ebay shop, as they have screw in Sorbothane feet as well and many others.
http://www.ebay.com.au/sch/m.html?_odkw=&_ssn=dang-good-stuff&hash=item56417b6303%3Ag%3AUA8AAOxy4dNS6cB0&item=370465792771&_osacat=0&_from=R40&_trksid=p2046732.m570.l1313.TR4.TRC2.A0.H0.TRS0&_nkw=Sorbothane&_sacat=0
Cement slab floors are the opposite, you want to couple the speaker into them via spike or cones.
Cheers George