I recall listening to a small pair of Salk Sound Veracity speakers at a show. They demonstrated their bass performance by cutting in a 12" Rythmik subwoofer. Those little speakers did very well on their own.
I managed to get much better texture from a large beautifully built $9K REL Stadium III locating it in the rooms main mode placing it on its side and slaving an optimized low level signal from another subwoofer.
Better than no sub at all is all I'd say about REL. Who rates subs at -6dB?
Forget the stereo. Crawl test and map your room for the two loudest modes. Place your Rythmik in, or close to one, and the REL in the other. Get two pairs of inexpensive long interconnects from Blue Jeans or Monoprice and use the L/R RCA outputs from the Rythmik to your REL. Now go beat your own drum.