nasaman,
REL Reference models, the No. 31 & 32 are sealed subwoofers that don't use a passive radiator.
I initially loved the Carbon Special. My listening room is upstairs, carpet over a suspended wood floor. I could hear the passive radiator and in my system/room it eventually drove me crazy.
The No. 32 is totally different. Despite their size, the No. 32s completely disappear, there's texture to the bass. It provides deep, articulate "musical bass", that's simply wonderful in my system. My wife recently confessed that the No. 31s were worth every penny to her. My results doesn't mean that everyone will arrive to the same conclusion.