Subwoofer recommendation for Dynaudio Confidence C4

Hi, 
     I'm thinking of adding 2 subwoofers to my Dynaudio Confidence C4. Leaning towards REL S/5 pair. Is this the right choice for Dynaudio?  Do I need 2 sub's? Does anyone see any issues with this setup? My AMP is Bel Canto e600 Mono block.