Distributed Bass Array configuration

I hope a sub with a continuous phase control will work well enough.

Nope.

"All-Pass phase controls operate at only a single frequency with varying degrees of shift across the passband."

see http://www.ielogical.com/assets/SubTerrBlues/PhaseControl.png at  ieLogical SubterraneanHomesickBlues for an image of how a continuous phase control functions.

Mike_in_nc, if you get an acoustic measurement mic and program like this one, https://www.parts-express.com/Dayton-Audio-OmniMic-V2-Acoustic-Measurement-System-390-792
You can measure group delays and frequency response. You should be able to adjust your subs perfectly.  I assume with their location you are using a low crossover point. What are you using for your main speakers?
Where you place them depends on your main speakers. Subs always perform best in corners. They are up to 9 dB more efficient which means much less distortion. For point source main speakers four subs in four corners would be ideal but two in corners and two right up against side walls would be a close second. A sub should never be in the middle of the room. That is like running your outboard motor out of water. For maximum dynamics you have to be able to adjust delays so the sound from your subs gets to the listening position at exactly the same time and in phase with the sound from your main speakers. This requires digital bass management. Are you still using a digital crossover? What happened to the 2.2X? Yes, Apogees have a reputation they do not deserve. They were capable of amazing sound on occasion but their fragility disqualifies them. Magnepans are a much better design from a reliability standpoint and if a ribbon tweeter blows (which they do rather frequently) Magnepan has a great tweeter replacement program and if you can use a screw driver you can change the tweeter. Apogee went belly up for a reason. 

Don't know if you are familiar with  Earl Geddes?

You should check him out and read the several white papers regarding subs and other stuff

http://www.gedlee.com/Papers/papers.aspx

@danager -- Thanks for the link to the Geddes papers. I surely know the name, but I don’t think I’ve read anything of his.

@mijostyn -- I use Janszen Valentina P8 main speakers. In my basement room, they respond well to 20 Hz, but a room-induced hole at 40Hz necessitates subs. As you say, subs project more authority at LF, even though SPL may be the same. The two Fathom F112 subs are in the rear corners, next to floor-to-ceiling bass traps. That is the spot in which the 40Hz hole can be filled most reliably. I use digital crossovers at 80 Hz, mostly, done by an Anthem STR Preamp, far better sounding than the Classé CP-800 it replaced.

I used to adjust sub delays with an OmniMic2, but that broke. (Lesson learned: do not position measurement mics by grasping the wand and moving it.) In a stroke of fortune, I won a UMIC-2, so I am learning REW, which appears quite capable, if complex.

I sent the TacT 2.2X to a mod shop in southern California to have the SRC replaced with a more modern one and some other components swapped. Unfortunately, the proprietor never finished the work and never returned the unit (which five years ago, was in pieces in a storage locker somewhere). The Anthem is one of the newest generation of audio DSP devices, and unlike the TacT, and very unlike the Classé, I don’t hear any artifacts when engaging the DSP.

     I completely agree that Duke Lejeune of Audio Kinesis is the most knowledgeable. best and most credible source on these forums about all things concerning 4-sub DBAs. He's always willing to assist and has probably forgotten more about proper 4-sub DBA systems and setup, as well as how bass sound waves behave in domestic sized rooms, than I've ever known. He often chimes in on threads like this but, unfortunately, not this time.   I agree with clio09 that you should call or email Duke at Audio Kinesis for the best advice.  Best wishes and enjoy, you're going to be amazed once your DBA is setup properly.
     Here's a little history on my experiences with the DBA concept for those interested:
      I actually first learned of this exceptionally good bass concept by chance.  Over 10yrs ago, I was just looking for the best way to supplement the bass performance of my combo music and HT system that utilized Magnepans as mains. I had called Audio Kinesis about another matter, talked to Duke's associate, James Romeyn, and we got talking and emailing about my system goals.   He suggested either of their two 4-sub DBA versions (the AK Swarm or Debra) as a very good bass solution for my Magnepans.  
     This was the first time I'd heard about the 4-sub DBA concept and it really piqued my interest. I dove into reading all I could find on-line about the subject, beginning with Earl Geddess's PHD Thesis on the concept, his subsequent White Papers on it, articles on multiple subs by Floyd Toole and Harman's Todd Welti as well as several others.
      At the time, I was planning on buying a pair of high quality subs such as JL Audio, REL or Vandersteen for up to about $6K.  But I decided to give the AK Debra a try instead since this whole scientifically tested and proven concept of the 4-sub DBA made so much logical sense to me, I was very interested and curious about its practical performance, I felt a bit adventurous and wanted to try something new, interesting and nonconventional.  
     Anyway, I was amazed, and still am a decade later, at how well this DBA concept performs in my 23'x16' room.  It took half a day to properly set it up but I didn't care once I heard and felt the results. It performs way beyond my expectations on both music and HT and is definitely the single best upgrade I've ever made to my system during my 50+ yrs in this hobby.   Hyperbole?  Maybe, but this concept is outstanding and I know it will work equally well with any main speakers I care to utilize now or in the future.

Tim