I'm using a single Rel B3 with my Dyn C1 sig's. I use the sub just to pick up what the C1's can't do. But I will say with Rel do not use the stock hi level cable. It takes away from the attack and decay in the sub bass notes. I only have about 6 tracks that the Rel adds the sub bass the C1's can't do. The S1.4's are great speakers (little more lively than the C1's).
That being said the sub will fill out the mids and add to the soundstage when there is no sub bass.
That being said the sub will fill out the mids and add to the soundstage when there is no sub bass.