I just switched from one Martin Logan Abyss to a pair of Emotiva X-REF10 subs. I never really made a huge effort to dial in the Abyss; that said, I'm not done with the options for doing so on the X-REF10.
I started with a single X-REF10 to get some idea of the room modes and best guess for crossover frequency and phase settings. After that, I installed the 2nd sub. Both are symmetrically located on either side of the centerline of the front wall, maybe 1/3 the way in from each sidewall. (My room is 22' x 13' x 9', and my system is on one short wall.) The difference with 2 subs, while not huge, was definitely noticeable. HOWEVER, I use small monitors (Von Schweikert VR-1) and I have the subs crossed over at a high frequency (125 Hz). So, it's not surprising I can tell the difference between a single mono sub and a pair in stereo.
As others said, depending on your mains and your crossover method, it may not make a big difference to go with a pair of subs.
I started with a single X-REF10 to get some idea of the room modes and best guess for crossover frequency and phase settings. After that, I installed the 2nd sub. Both are symmetrically located on either side of the centerline of the front wall, maybe 1/3 the way in from each sidewall. (My room is 22' x 13' x 9', and my system is on one short wall.) The difference with 2 subs, while not huge, was definitely noticeable. HOWEVER, I use small monitors (Von Schweikert VR-1) and I have the subs crossed over at a high frequency (125 Hz). So, it's not surprising I can tell the difference between a single mono sub and a pair in stereo.
As others said, depending on your mains and your crossover method, it may not make a big difference to go with a pair of subs.