I too had this question and found the solution with 2 subs. I use Merlin TSM-mme speakers. The sound is amazing, but pipe organ pedal notes and very low bass were just not going to happen.
I purchased one subwoofer and carefully dialed it in. The payoff was better sound, but the system became somewhat directional. The Merlins did not disappear as well before the subwoofer.
I bought a second sub. I placed one sub next to each speaker stand and carefully dialed them in. Now I get the Merlin magic and low bass. My experience is that 2 subs are better than one. Make sure you use the same model subs and they are very high quality.
I purchased one subwoofer and carefully dialed it in. The payoff was better sound, but the system became somewhat directional. The Merlins did not disappear as well before the subwoofer.
I bought a second sub. I placed one sub next to each speaker stand and carefully dialed them in. Now I get the Merlin magic and low bass. My experience is that 2 subs are better than one. Make sure you use the same model subs and they are very high quality.