If you're into rock / metal then a sub might help your setup (subs also add a lot to full orchestral classical pieces ... more than you might expect). Try either REL, or perhaps home demo an ACI titan (which many seem to like, and works in a pre-power amp setup). I have a REL strata 3 ... it has transformed my system, and now I can listen to choral music, chamber music and rock and it all sounds great.
A sub frees you up from having to purchase true full-range main speakers, and it also allows you to place the mains optimally for soundstaging without having to worry about bass reinforcement. (Of course I'm not considering the WAF in the speaker placement). Also you can choose mains for their soundstaging and tonal characteristics, rather than having to settle for something with the last octave in bass.
However I wouldn't add a sub if the mains are already a bit bass heavy.
A sub frees you up from having to purchase true full-range main speakers, and it also allows you to place the mains optimally for soundstaging without having to worry about bass reinforcement. (Of course I'm not considering the WAF in the speaker placement). Also you can choose mains for their soundstaging and tonal characteristics, rather than having to settle for something with the last octave in bass.
However I wouldn't add a sub if the mains are already a bit bass heavy.