1) Deeper, more visceral bass.
2) At present, I'm confined by "domestic considerations" as to sub placement and size, as my system is located in a Living Room rather than a dedicated listening room. (The room is approximately 14 x 24, with two doorways, woone double-wide, and a staircase to the second floor.) The REL is located in a corner, along the same wall as a pair of Sonus Faber Maxima Amator. I was thinking that a higher-powered sub of similar size might make an impact. That and the Fathoms have built-in room correction software.