an isolation pad prevents the sub from transmitting its vibrations to your home structure.
wal-mart sells a foam "seat" cushion for hunters that is about 1.25" thick, 18" wide and 14" deep. it is made to insulate hunters butts from cold rocks or snow, but it will also minimize vibrations from your sub to the floor. costs about $10.
if you want to spend more money, i think there is a product called subdude or something like that. about $50+ for that one...http://www.auralex.com/sound_isolati...de/subdude.asp
available on amazon...
you could probably visit the dumster of a carpet store and get all the padding that you need to decouple your sub from the floor for free if that is your thing.
the theory is that you do not want your sub to vibrate your structure because it wastes a whole lot of energy and creates annoying resonances. dual opposed wont vibrate the structure, neither will subs built with heavy cabs and light drives. if using a light enclosure and heavy drives, you may get some benefit from decoupling the enclosure from the enclosure, but its a cludge, not an optimal design.
as a side note, the whole "spiked" mains design was to couple the enclosures to the floor in order to minimize the vibrations of the cabs, which in theory gave better midrange performance.
when monster subs came on the scene, this idea got thrown out, as the vibration coming from the enclosure could not be held be the floor and it created all kinds of resonances and vibrations in the structure.
the response solution was to simply decouple it from the structure.