Actually if you want to "really" take it to the max,there is the Eminent Techology TRW-17!!
An experienced audio friend had this demoed for him,and was flabbergasted!!
This "is" truly a serious design,but requires a commitment from the hobbyist....as in re-doing some vent work in the room/house.
It is reviewed on the "IAR" web-site,where it is proclaimed "the only subwoofer".
Looks serious,but I have not heard it and never will.I've got alot of experience with REL's better offerings and do think they work well.
The room size,and what the hobbyist realistically expects(aside from the financial considerations)will dictate the ultimate success.
Good luck....you already have a great speaker-:)