Stereo Subwoofer Pairing

" Deep bass is not directional, and that has been shown by a lot of research. Yet, as some have argued here, it may sound as if these deep bass notes do indeed emanate from different speakers. The explanation is that the human brain uses the upper harmonics from the main speakers to locate the deep bass tones."

Hi willemj,

     Excellent post that really clarifies the forces at work.  I was starting to think that deep bass may be directional because I'm able to perceive the locations of deep bass instruments within my sound stage illusion in a very consistent and convincing manner. 
     Your explanation, that the human brain uses the upper harmonics of the deep bass emanating from my main speakers to locate the deep bass tones within the sound stage illusion, really helped me understand the forces at work and the reasons for my confusion. All 4 of my subs are run as mono signals and mentally making sense of the deep bass localization that I am perceiving was getting difficult.  Our brains may be the most valuable component in all our systems.   It's all clear to me now,  Thank you.
      I'm not aware of the OP's, rjb1101's, room size and budget so I'm not sure if he even has the space or funds for the ideal solution of a 4 sub distributed bass array system but agree that 2 subs are better than 1 as many have already stated.  
     Assuming he wants to use just 2 subs , I think it's more important for good bass response that there are 2 subs instead of 1 than it is that they match. 
     If  he's mainly concerned with good bass response at his listening position,  I'd suggest he run them as mono and position them in the following manner:

1. Position sub#1 at his listening position and play music with good and repetitive bass.
2. Slowly walk across your front wall from right to left and find the exact location that the bass on the music sounds best to you and place sub#1 at this spot. If you prefer accurate and natural bass, avoid placing sub#1 in a corner.
3. Place sub#2 at your listening position and play the music with the good and repetitive bass again
4. Slowly walk along your left wall, from front to back, and find the exact location that the bass on the music sounds best to you.  Place sub#2 at this spot again avoiding corner placement if possible.
5.  Sit at your listening position and play the same music again.  If the bass response is not very good, start again from the beginning.  Remember, even small adjustments in positioning can affect bass performance at your listening position.

    In most rooms, 2 subs perform best with an asymmetrical positioning scheme; that is, with one along the front wall and one along a side wall rather than symmetrically with both placed along the front wall with one placed behind each speaker. 
    If you really prefer both subs along your front wall behind your speakers, however, you could try this by substituting the following for step #4 above: 
   Slowly walk along your front wall, from sub#1 to the left, and find the exact location that the bass on the music sounds best to you. Place sub#2 at this spot again avoiding corner placement if possible.
     The only way you'll discover which positioning scheme sounds best to you in your room is to try them both. 
     I'm almost certain that you'd get better bass response in your room, and certainly better bass response throughout your entire room and not just at your listening position,if you utilized 4 subs in your room rather than 2,  but you may have constraints I'm unaware of.  I hope this post was helpful to you.
  
Best wishes,
   Tim

FWIW
I employ a pair of inexpensive Andrew Jones powered subs with either my Sonus Faber Cremona Auditor M Monitors or Magnepan .7 speakers.

One sub is in the right corner of the front wall, the other in the left corner of the rear wall. I experimented with numerous placement arrangements and after a great expenditure of time I decided that this basic configuration yielded the best results. It works quite well.

Sometimes satisfying home audio is not beholden to the number of angels which theoretically can fit on the head of a pin. Simple is often best.

I use 2 differing REL models from the same era (Q150e, Q108 MKII) wired together getting their signal from the main tube amp. With bass I've found that the tone indicating where a bass is in the mix comes from whatever higher frequency overtones (or simply tones) are there so they're basically mono in their low range duties. 2 subs work better though to smooth out room nodes as perceived from the listening spot, with one sub (the larger Q150) right behind the left speaker, and the other against the right wall so I can stick it in a window for deck listening (with a couple of monitors also stuck in the windows powered by another amp). I agree that keeping the RELs lower (but properly matched to the mains) with higher gain works best. 

The Antimode engineers recommend corner placement for maximum reinforcement and minimal dips/nulls. That position unfortunately also gives the biggest peaks, but you can deal with those with the room eq.

" The Antimode engineers recommend corner placement for maximum reinforcement and minimal dips/nulls. That position unfortunately also gives the biggest peaks, but you can deal with those with the room eq."

Hi willemj,
     
     Yes, I've found that corner placement of subs results in the most bass due to the long bass sound waves reflecting off of both walls in a corner. This often results in bass peaks (exaggerated bass) at spots in the room. That's why I advised against it although some may like it.
     I've found avoiding placing subs in the corner results in more accurate and natural bass response which I prefer.
    I remember you mentioning your Antimode DSP room eq unit before .  I  use zero DSP, eq and bass room treatments in my room since I haven't found a need to since employing the 4 sub system. It's almost magical how this system eliminates bass nodes in the room and just provides accurate, natural and, when the source calls for it, very deep  and powerful bass on both music and ht.