It's nothing to do with REL, and all REL does with stuff like this is reinforce the perception of being first and foremost aggressive marketing driven.
The pertinent question is how does really good deep bass reinforce the perception of dimensionality and space?
The answer is by believably recreating not only the sound but the feeling of being in a large space. Very low frequency bass waves are very long, 40 to 50 feet or more. Much larger than any normal room. Normally such low frequencies are only experienced in large or open spaces. So when we hear and feel them we naturally feel we are in a large space.
This has the effect of making room walls disappear, creating a feeling of envelopment, of being in the original acoustic space and not just having it up there somewhere in front of us.
Because this feeling of being an enlarged space extends off in all directions it creates a feeling of a larger space in front and to the sides as well. No wonder a recent visitor who has been here a few times now describes the sound as "cavernous". He's not talking about an echo. He's talking about a vast black space. This is where a lot of the dimensionality comes from.
Nothing wrong with REL, other than they are overblown. However good one or two REL are, any four decent subs will do this a whole lot better. Probably for less. That is the problem with the REL hype. Any four. Do a search. You will see.