A subwoofer is also a "speaker" (essentially), i.e., the same benefits one may reap from isolating a potent speaker shall also apply to a sub in a hifi application...unless the goal is to initiate boomerman mode, crack the drywall, bring the ceiling down, etc... like the hometheater dudes with a platoon of 18 inch subs.
Isoacoustics makes some good stuff for sub isolation that's cheaper.
If you want to spend more, Acoustic Fields can hook you up.