REL models typically have a passive radiator which makes their bandwidth a mixture of in phase and out of phase (back wave pulse) bass frequencies. This makes it easier to integrate them to the main speakers and not require a phase adjustment. It's all about having a crossover frequency and proper subwoofer position that eliminates or minimizes summed peak obnoxiously dominant bass notes.
Having had RELs and other subwoofers with continuous phase the RELS were a better solution overall but still had shortcomings.