Mixing REL subs, great idea or terrible idea?

It is a lot easier to mix subs if you have a miniDSP.

OP:  miniDSP is line level only. You'll need to get the signal before the amp.

ultrabright:
" @noble100 I have a love/hate relationship with the crawl method. I now am also very tired of the Sneakers soundtrack haha. The T7i definitely comes to life in the corners of the room. It was really cool to hear the difference positioning makes."

Hello ultrabright,

     Yes, placing a sub in a corner will increase the bass level in a room  because bass sound waves are omnidirectional, meaning the sound waves radiate out from the bass driver in all directions in a 360 degree pattern.  Placing a sub in the corner results in these bass sound waves having 2 wall surfaces to bounce or reflect off of, instead of just 1 wall when  the sub is placed along a wall and not in the corner, and back into the room.  
     All of the bass sound waves, radiating out omnidirectionally from all of the bass drivers in your room, continue in an outward direction into the room until they encounter a room boundary (ceiling, floor or wall).  Once these bass sound waves encounter a room boundary, they bounce or reflect off of the room boundary and continue to travel outward into your room in the reflected and redirected pathway until they once again encounter a room boundary and reflect, run out of energy and fade away or collide with another bass sound wave.
     Every time 2 bass sound waves collide in a room, what's called a 'bass room mode' is created at the exact spot of the collision in the  room.  We perceive bass room modes in one of 3 ways, depending on the exact direction and angle of the sound wave collision: 1. a bass peak (bass exaggeration), 2. a bass dip (bass attenuation) or a bass null (bass cancellation or absence).
    You can demonstrate this acoustic fact in your own room by playing some music with strong and repetitive bass and then walk around the entire area within your room while listening closely to the characteristics of the bass.
     So, there are 2 points you need to understand well if you want to attain good bass performance :

1.  You're going to notice many bass room modes at specific positions in your room. 
2.  Whether you have a love or hate relationship with the crawl method, using it to optimally position each sub in your room, is the simplest and least expensive way to ensure there are no bass room modes located at your listening seat that I'm aware of. 
     Automated room correction can be run afterward if you like,  but it's only really effective at correcting bass peaks and limited in its effectiveness at correcting bass dips and nulls, mainly due to the finite power of your amp.
     ultrabright:
"I imagine that with the four 12” woofers between my Spatial Audios combined with a second sub (for a total of 6 drivers), I should have plenty of low end to fill my 12 x 14 x 9’ room, despite it opening up into a much larger (twice as wide and 12’ ceilings) space to my left. I’m surprised at how the addition of just one sub was able to pressurize the room as is."

     That's not exactly how it works. I'm certain that utilizing 4 or more subs in a room, in what's called a distributed bass array (DBA) configuration, results in extremely good bass performance throughout the entire room, not just at a single designated listening seat.  I know this because I use this concept, and 4 relatively small 1'x1'x2' 65 pound subs in my own room, and it works extremely well throughout my entire 23'x16' room and the bass integrates seamlessly with my main stereo speakers.
     Yes, you have 4 or more bass drivers in your room but the DBA concept requires that each sub/driver be independently positioned in the room optimally by using the sub crawl method or at least asymmetrically positioned throughout the entire room. 
     Unfortunately, you're unable to do this because most of your subs/drivers cannot be independently positioned in your room, since most of the subs/drivers are permanently attached to your main stereo speakers and I'm assuming these main speakers have already been positioned in your room, and in relation to your listening seat, to optimize midrange, treble and stereo imaging performance.  Unless you're winning the lottery lucky, I seriously doubt bass performance is also going to be good at your listening seat.
     I suggest best overall results will be achieved utilizing 2 independently and optimally positioned subs using your beloved sub crawl method.

Best wishes,
    Tim

I had a T7 in one front corner of the room and the 528SE REL in the other.  They worked very well with each other.  Ideally, another 528SE would have looked better but they were very difficult to find. 
I probably won't buy any other brand sub in the future.  

Hello oduckfan,

      Yes, REL subs are generally considered very good subs, but there are a lot of very good subs available today that have the same controls and connection methods as the RELs, including the high level speaker connections, but are also less expensive such as SVS and HSU.
     Sure, you can plop a sub in each corner and call it a day.  You'll certainly attain more bass in your room doing this but you also certainly won't attain the highest quality bass.
     You can optimize the quality of the bass in your room by sequentially locating each sub utilizing the 'sub crawl' method.  Using the sub crawl method for positioning each sub, it will be obvious at which specific room location the bass sounds best to you, with 'the bass sounding the best to you' meaning bass that is powerful, dynamic, smooth, fast, detailed, accurate and natural.
     You'll also notice that the optimum position for each sub will be along one of the room walls but at least 2 feet away from any room corner. 

Tim 

OP, I currently run two R series with two SHO series REL subs. They sound great together. I also use what I will call the Duke Lejuene "swarm" method. Duke gets the credit because he graciously walked me through how he thought I should deal with bass issues in my particular room.