The Elac S12EQ has a passive radiator mounted on the bottom of the enclosure. Sometimes these radiators can be very heavy if the subwoofer is trying for a very low tuning. At high excursions, that could cause some vibration- and that would be a design problem, if that is the case. The subwoofer would need a more inert cabinet. As lovinthehd said, an isolation pad shouldn't be needed on a concrete floor. That would only make a difference if the subwoofer itself were the problem.