One or two subs

All things equal, more subs equals more efficiency and output capability. Smaller rooms have a problem both absorbing bass energy, however, for proper RT60, and long bass frequencies don't support well in diminutive dimensions. So I like small single subs in small spaces, multiple 8's placed together or single 10" sealed woofers in medium spaces, and maybe 12" dual's or better stacked together, or flanking each channel as stereo subs in large spaces.
Problems you run into with spacing multiple (in this case, two???) subs, spaced 1/2 way points along the wall boundaries: PHASE CANCELATION at different seating locations! Sit closer to one sub vs other in a room with subs run in mono, and you'll have a good ol time trying to not get phase cancelation issues between the two! It's simple acoustics.
Pick the sealed subs in small and medium sized rooms, and go multiple subs for more output and dynamic range from full on HT duty applications! ..but keep em together, say, in center of front of the room, near the main center for best crossover, easier integration, and lack of phase issues from seating locations. Blends best, I tell ya.

When it comes to subs, two is always better than one in terms of better and more consistent sound quality throughout the room, the caveat though it may take longer to get two subs mixed in optimally compared to one.

The more, the merrier in general, though with diminishing returns of course. Even 4 or more is not out of the question in some cases.

Once the bass levels are right no matter where you listen from in the room that you might care about, you are done.

If you listen from one location only, and bass levels are fine, then one sub may suffice just fine.

With one sub (ie no stereo), its more beneficial than otherwise to configure so sub fill in solely in lowest octaves that mains cannot handle otherwise in that sound becomes more directional at higher frequencies.

I had two subs and used a Dspeaker 8033s ($400.00) to control the room nodes. I'm now using the Dspeaker Dual Core 2.0 and can't believe how good the bass sounds.

Just my 2c

I've been using my two Velodyne HGS-15s with an SMS-1. Since I acquired a second SMS-1, I have been reading the manual about daisy chaining them so each sub can have its own acoustic room correction. I think I'll give it a try.

db

Guys, thank you for all your responses. Ive decided to go with two Rel T7s. My pre is the yamaha cxa5000, has dual sub outputs but the second output saids for rear sub, should i just plug into that or get a RCA splitter, thanks and will update once i set everything up.