The REL Stentor 3 and Studio 3 subwoofers (probably also the older models) are loaded with a folded transmission line which REL calls a "Super Acoustic Resistive Matrix".
Most PMC speakers make use of the LT principle but I do not know whether or not this is also the case for their subwoofers.
Most PMC speakers make use of the LT principle but I do not know whether or not this is also the case for their subwoofers.