It is much easier to get a flatter response curve at multiple points in the room with multiple subs, carefully placed.
When I used a single sub (an eD A7-600 with dual 12" drivers - is that still a single sub?) I could get measured in-room extension to 20Hz and below, but the measured response curve was far from flat and varies from seat to seat to seat. One can equalize to help somewhat, but even then you're fighting a battle of averages, and the result will always be a compromise.
I use three subs now, which (when carefully placed) significantly reduced the peaks and valleys in my frequency response curve, and showed much less variation when measuring at different listening points in the room. From there, I still use a Behringer DSP in a dual-channel configuration to further tame that response. The resulting curve is VERY close to flat, and much more uniform when measured across different seats.
One single sub (even if it's the world's greatest sub) may very well get you both the volume and the low Hz you require, but it won't get a flat response, and it will vary at different points in your room, even after you equalize.