I tried one subwoofer, but I always had the impression, real or not, that I wasn't getting true stereo. So I added another. Both were positioned below or next to the main speakers because I wanted the wave front to be as coherent as possible - which doesn't happen when the mains and the subs aren't in the same areas. Anyway, I continued to use the subs when I got my Martin Logan ESLs until one day when I disconnected them and found the sound much improved. The subs (actually sub, since on was blown by a recent power outage) are now working with a pair of Celestion SL6si's.
Move your sub
Caught an Art Dudley video about moving your sub(s) from behind, beside or between your speakers to the seat end of the room. I moved my REL back so they are in the same plane as the listening position. Much better! Try it.