My SVS 25-31PCi is on a hardwood floor with just the standard feet and it has stayed quite still (although I can't say the same for some knick-knacks in the vicinity...). The idea of a slab of something heavy underneath is a very effective for vibration isolation, but I don't know if it'll stop the sub from hopping unless you are getting floor movement of some sort. Zacster's right, SVS can probably square you away one way or the other.

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