I have two Mariana 24" sealed subs which use the 24" SI drivers (not sure if I have the mkii or mkiii) version. Each sub has a built-in SpeakerPower 4000W amp. My room is 22'x15'x8' (about 2/3 the volume of your room) on a suspended wood floor. The subs are located on the mid-front and mid-rear walls. The MLP is about 2/3 between the front and rear walls.
As far as ULF power and extension, two 24" subs are plenty for me. I get plenty of 'warble' and room pressurization if that's what you're looking for. What's lacking for me so far is mid-bass chest slam, but I'm still tweaking my room placement, room treatments, and phase/frequency EQ using a minDSP-HD to tune each sub independently.
IMO, subs at this level are desired not only for low-frequency sonic (SPL) capability, but also for good Tactile Response (TR). I divide TR capability into two general categories which are ULF warble (a steady-state vibration starting at 30Hz going down to single-digit Hz), and slam (a sharp transient impact driven by frequencies around 50-80Hz). In general ULF warble can be generated by sealed subs with large displacement, or ported subs with low-frequency port tunes. A suspended wood floor also helps with ULF warble. If you have a concrete floor, simply adding isolators under your seating will also help (see BOSS link below). Chest slam OTOH is most easily produced by near-field subs (within a foot or two of your listening position), or large diameter subs a few feet away. In either case, careful tuning and time-alignment is needed to achieve good chest slam.
If two 24" subs don't give you enough ULF warble, my recommendation is to add transducers under your seating such as Crowsons, and/or BOSS risers/mini-risers
. That way you can get all the ULF movement you can stand without having to crank the subs that could possibly damage your house or hearing, or at the very least, offend neighbors or family members in other rooms.
Hope this helps,