Sub. with full range speakers ?

I've heard pretty much the same improvements when listening to friends systems when adding subs to full range speaker systems. Much better soundstage depth and better imaging. Someday I hope to add one of the Thiel subwoofers to my 7.2s.

Will one Rel B1 sub work with a system. Do you realy need stereo subs. ?

I have been asking myself the same question. Here is what may be worth considering...Space, the final frontier...size of room...the larger the room with open adjacent areas need more cubic air movement by the sub to get the same "coupling" effect than in a smaller enclosed room. If acurrate individual hall signatures or ambience is a priority then two subs may be in your future. Crossover and interaction with both speakers & room vary alot. Some people are more sensitive to "background bass" as produced off in some corner as opposed to "time aligned" which requires either tweeking the signal in the time domain electronically or simply placing the subs in a stereo position closer to the main speakers or even slightly ahead of them...
The Hafler effect also may be worth trying on front firing or other type subs. One sub across the room/hall from the other set for opposite phase. Like a push-me pull-you arrangement.
I have been considering getting a second REL B1 (front firing) and actually using the subs as speaker stands for my main spks. Of course this requires moving the existing out from the corner and making sure the tweeters are correctly aimed to the new elevated listening position...
Enjoy your Music!