Two. I've worked on that solution over ten years ago and after many, many hours of tweaks, found that two, with proper placement does the trick. I run them with full range speakers PROAC. So, look into two. I have NHT units with outboard amps. Oh, the joys of improving your listen are endless!
One sub or two or none?
The question is-one sub or two-or none? Some purists may say E-Gads man-NO SUBS. Each to their own. Since I became aware, I have liked a lot of bass. It was always a test to see what speaker put out the most low frequency. I don't know why I didn't think of the "sub bass" idea long before it became popular. Guess it's like the invention of the wheel-once someone came up with the idea, everyone said "of course." Anyway, I take my sub use on a more subtle basis now. Some well produced LP's/CD's have all the low frequency you may want; sadly others, the bass was rolled off for different reasons. Back in the day, the bass was rolled off on rock because many didn't have the equipment to reproduce it. After all, most listened to AM radio. Even when FM became the modulation of choice, the bass was rolled off around 80 hz or above. On classical recordings, the full frequency was available for all to enjoy. Good for them. Anyway, I have tried two subs, then one. I made a couple subs that incorporated two JL 15 W-1's, pushed with a 600 watt RMS amp. I even connected a dbx sub-harmic synthesizer to extend the bass. The sound was good, but lacking. I finally purchased a JL Fathom 113. Finally, I had all the bass I wanted at the proper frequency. I set the crossover to 40hz and below. Then it was where to place it. Jim Smith's book How To Get Better Sound, says two subs are better than one. In my experience, one is as good as two. In articles written about two subs, some say to stack the subs to reinforce the bass so they don't cancel each other out. Even then, you have to be sure they're in phase. I tried one positioned just to the left of my right channel speaker, then in the center. I have to say the center gives the best sound for me. OK-some are saying Hello-that should be obvious. However, most say low frequency is not directional, so placement isn't critical. I can tell a huge difference, mostly with LP's when it's placed in the middle. Before stereo, theaters achieved better sound by placing the low frequency speaker in the middle of the theater. I'm thinking this is still a good idea. Partly because you cut your speaker cost in half and partly because two are not better than one. This opinion may be worth what it cost you. Try it yourself-that's the ultimate test.