anyone hear the REL Carbon limited Subwoofer for 2 channel audio

A good friend has a pair complementing Revel 228bes, they sound fantastic together. The Revels have amazing bass on their own but a pair of carbons really took his system up a notch for sure.

I forgot to mention that I am using a pair of the Carbon limited, one to the outside of each speaker.

I hesitate to respond since I have no basis for comparison, but I am using the REL Carbon limited with a pair of Martin Logan CLXes and I think that they are wonderful.  They add bass impact and depth to the speakers that was otherwise absent.  Don't get me wrong, I think that the CLXes by themselves have quite adequate bass, at least in my room, but the REL's add another dimension for the texture and color of instruments like trombone and acoustic bass.  In some cases, this extra dimension can be mesmerizing.  When I first placed the RELs in my room, I could not believe how much more realistic the acoustic bass sounded on an old Bill Evans album with Eddie Gomez on the bass.  The dealer that I purchased the CLXes from thought that the S series would work well with the Carbons, S212, and SHO-5 in that order of preference.  I also like that the Carbons do not draw attention to themselves visually although I think they are quite beautiful.